doi:10.1144/GSL.MEM.2003.020.01.68 2003; v. 20; p. 811-824 Geological Society, London, Memoirs

 A. Moscariello  

The Schooner Field, Blocks 44/26a, 43/30a, UK North Sea 

Geological Society, London, Memoirs

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The Schooner Field, Blocks 44/26a, 43]30a, UK North Sea

A. M O S C A R I E L L O

Shell U.K. Exploration and Production, Lothing Depot, Commercial Road, Lowestoft NR32 2TH, UK Present address: Shell International Exploration and Production, Volmerleen 8, 2280 AB Rijswijk, The Netherlands

(e-mail: a.moscariello @shell. com)

Abstract" The Schooner Field is Shell U.K.'s first Carboniferous gas development in the North Sea. The field was discovered in 1987 by well 44/26-2 and gas production began in October 1996 from four wells. In contrast to the majority of the fields in the Southern North Sea producing from the aeolian Leman Sandstones Formation (Rotliegend), Schooner targets the low net-to- gross, fluvial Upper Carboniferous Barren Red Measures and Coal Measures formations. The reservoir consists of discrete, low sinuosity fluvio-deltaic channels draining a swampy coastal floodplain evolving upwards into a highly aggrading, low gradient, distal fluvial fan, dominated by braided and anastomosing channels. In Schooner, like other Carboniferous fields, reservoir connectivity is one of the key subsurface uncertainties due both to channel lateral discontinuity and fault compartmentalization. Production data and reservoir properties distribution, together with a new stratigraphical subdivision driven mostly by chemostratigraphic techniques, have been used to reassess the volume of gas-in-place which currently is estimated at 29.98 Gm 3 (1059 BCF).

Location

Schooner is Shell UK's first Carboniferous gas reservoir to be developed and is located in the Silver Pit Basin approximately 150km off the South Yorkshire coast (Fig. 1) within Shell/Esso concession Block 44/26a (licence P516) and Eastern Energy/Cal Energy Block 43/30a (licence P689). Shell operates the field on behalf of fixed-equity partners (Shell 43.8%, ExxonMobil 46.55%, TXU Europe Upstream Ltd 4.83% and Cal Energy 4.82%, on November 2000). Licence expiry is 13 June 2021 for P516 and 2025 for P689, with Production Consent until 31 December 2014.

History

sented by the evaporitic lacustrine shale of the Silverpit Formation (Fig. 2), would provide an adequate top seal for gas accumulations within the underlying Carboniferous. Up to 1983, only three dry exploration wells had been drilled.

Permian reservoir sands of the Leman Sandstone Formation were predicted to be absent over the area, although a thin basal Leman Sandstone was thought to be present to the south. Carbon- iferous fluvial and fluvio-deltaic sands formed the prime explora- tion targets in this region (Fig. 1) whilst structural closures at Triassic Bunter Sandstone level formed a secondary objective.

These new concepts, combined with the de-regulation of gas prices in the UK, made the Silver Pit a prime area of industry inter- est in the 8th, and subsequent, licensing rounds.

Pre-discovery Discovery

The Silver Pit Basin was largely neglected as an exploration area in the 1960s and 1970s due to the depth and the absence of thick aeolian reservoir facies of the Rotliegend Group, which forms the main gas reservoir in the areas to the south. By the late 1970s it was understood that the Rotliegend Group, which in this area is repre-

Block 44/26a was acquired by Shell/Esso to test a large structural high characterized by a faulted dip closure mapped from seismic at Top Carboniferous beneath the Permian Saalian Unconformity.

The discovery well, 44/26-2, drilled in June 1986, found a total of 102 m (335 ft) of gas pay in the Upper Carboniferous Barren Red Measures (BRM) Group and 249 m (816 ft) in the Coal Measures (CM) Group. The well reached a total depth of 13590ft TVDss penetrating the gas-water contact at 13075ft TVDss. Reservoir pressure was measured at 6564psi. The BRM section flowed gas at a rate of 27.8 M M S C F / D at 3220 psi Flowing Tubing Head Pres- sure. Reserves estimated after the 44/26-2 well were 518BCF. Analysis of the test results showed fairly good well deliverability with good permeability (c. 100mD) for the channel sands and 0.05 mD for the thin, overbank sands. All the tested wells were partially per- forated and showed good well productivities.

The structure was subsequently appraised by well 44/26-3 in 1987 which tested 10.1 MMSCF/D. Well 44/26-4 in 1988, drilled 2 km to the north of the discovery well within the same structural closure (Fig. 3), was targetted at a deeper middle Coal Measures sand objective (the main reservoir in the Murdoch and Caister fields, 25 km further north) to fulfil the 16000 ft TVDss commitment. The primary objective was found to be tight and water-bearing.

Post-discovery

Fig. 1. Geographical location of the Schooner Field (Silver Pit Basin).

The Schooner Field was covered by a 300 km 2 3D seismic survey in 1988, which was processed during 1989. During 1994, the 3D data set was reprocessed and a reinterpretation and mapping project was conducted in early 1995 in support of the then imminent drilling of the first production wells. Although the reprocessing resulted in improvements, the conclusion was that the data set was inadequate for proper imaging of the subsurface. The data indicated a high

GLUYAS, J. G. & HICHENS, H. M. (eds) 2003. United Kingdom Oil and Gas Fields, Commemorative Millennium Volume. Geological Society, London, Memoir, 20, 811-824. 811

812 A. MOSCARIELLO

(a)

Fig. 2. (a) Stratigraphy of the Silver Pit area over the Schooner Field and (b) summary of chronostratigraphy, depositional and tectonic setting of the Westphalian Barren Red Measures and Coal Measures (compiled from Besly 1990; Leeder & Hardman 1990).

degree of uncertainty associated with fault mapping in the Carbon- iferous level. This also highlighted the potential high risk of drilling through the Zechstein Group, which is populated with potentially over-pressured Haupt Anhydrite/Platten Dolomite rafts (Fig. 4). A seismic inversion study undertaken in late 1996 highlighted the difficulty in interpreting the intra-reservoir geometries caused by the complex overburden and the poor quality of the original data set. This new study could only provide information of rather limited confidence on reservoir facies distribution. The interbedded gas bearing sands are typically in the order of 30 ft thick and are thus beyond seismic resolution, particularly given the poor dataset.

In early 1997 a total of 246 km 2 of new full fold 3D seismic was acquired and pre-stack depth migration was performed. The pre- stack depth migration (PreSDM) interpretation yielded a improved definition of the reservoir rock volume and fault geometry and dis- tribution throughout the field (compare Fig. 3 with Fig. 5).

The first development drilling phase started in May 1995 from the normally unattended SA platform. The wells were designed with deviations within the reservoir of approximately 50 ~ , in order to optimize net pay intersection. Completion strategy (e.g. inflow inter- vals, tubing size) is determined on the basis of gamma ray (i.e. sand prone intervals) and resistivity log data.

Discovery method

Structure

The Silver Pit Basin is a loosely defined area situated to the north of the main Rotliegend Group (Permian) gas fields of the late Cimmerian Inde Shelf and the late Cretaceous to Tertiary Sole Pit

SCHOONER FIELD 813

(b)

Fig. 2. (continued)

Inversion Zone (Glennie & Boegner 1981; van Hoorn 1987; Cor- field et al. 1996). The basin is separated from the offshore Durham Shelf and the Cleveland Basin Inversion Zone to the west by the Dowsing Fault Zone (Fig. 6). The Variscan Mid-North Sea High

Fig. 3. Structural map and cross-section of the Schooner Field reservoir based on 1988 3D seismic survey. GWC, gas-water contact.

defines the northern limit, while the Cimmerian Cleaver Bank High forms the southeastern limit.

The Silver Pit Basin developed in an equatorial to sub- equatorial position north of the then active Devonian to Carbon- iferous Hercynian orogenic belt. The basin was strongly influenced by this orogen and its northward migration. The area suffered litho- spheric extension in late Devonian to mid-Carboniferous times (Corfield et al. 1996). Active fault-bounded half-grabens and tilted fault blocks developed along a dominant NW-SE grain, succeeded in the Upper Carboniferous by a post-rift phase of regional sag, caused by thermal re-equilibration (Leeder & Hardman 1990). This resulted in the creation of two lowland areas separated by the NW-SE trending Murdoch fault system. The Schooner Field lies immediately south of this high (Fig. 6)

Variscan tectonism deformed the Upper Carboniferous strata by both folding and faulting along a dominant NW-SE fault trend. Seismostratigraphic interpretation, along with well control, indicate that early-formed basement faults at least intermittently controlled the location of channel belts during the deposition of the Upper Car- boniferous. Uplift and subsequent erosion associated with the Saal- ian Unconformity resulted in a pre-Permian subcrop ranging from Namurian in the west of the basin to Westphalian D (or younger?) Barren Red Measures in the east. However, the limited regional well data make it difficult to determine whether this was a distinct depo- centre during Carboniferous times. Since at least the early Permian, the Silver Pit Basin has been a centre of regional subsidence.

The effects of uplift associated with the early Cretaceous late Cimmerian tectonic phase were relatively minor in the basin with the result that a thick sequence of overlying Permian to Triassic sedi- ments has been preserved in the area. Late Cimmerian erosion was limited to the removal of the Jurassic and part of the upper Triassic strata. Halokinesis was initiated by these late Cimmerian move- ments and continued into the early Tertiary. Late Cimmerian reacti- vation of the Variscan faults, together with Tertiary Alpine wrench movements along NW-SE trending basement fault zones, resulted in the formation of tilted fault blocks at Saalian Unconformity level. These are usually bounded by complex reverse faults and form the principal proven gas-bearing structures in the Silver Pit Basin.

Tertiary Alpine tectonic activity has strongly deformed the post-Permian sequence into a series of anticlines and synclines with a dominant NW-SE grain, coincident with the major pre-Zechstein fault trend. Despite local halokinetic effects, the Silver Pit Basin continued to develop as an overall subsiding depocentre into which

814 A. MOSCARIELLO

i I ~ i I

Fig. 4. Seismic and geological cross-section over the Schooner Field from the 1997 3D seismic survey. The rafts within the Zechstein Group often consist of over pressured Haupt Anhydrite/Platten Dolomite couplet which have been broken and displaced during tectonic-induced halokinesis.

the Upper Cretaceous Chalk and Tertiary sequences thickened slightly. To the west of the basin, fission track analysis has indicated significant Tertiary uplift in two phases (Alberts & Underhill 1991). This has resulted in erosion of the whole Jurassic and most of the Lower Cretaceous sequence.

Local structure. The Schooner Field is an elongate NW-SE trend- ing anticlinal closure bounded to the SW by major NNW-SSE high- angle transpressional oblique-slip faults (Fig. 6).

The structure is believed to be the result of tectonic inversion of Cimmerian and/or Tertiary age and formed by uplift along a major reverse fault trend that is probably of Hercynian origin. The closure is 16km (10miles) long by 4km (2.5miles) wide, with the crest slightly offset to the SW. Within the structural closure, the Car- boniferous strata have been deformed into a broad SE plunging anticlinal swell. The main reservoir, the alluvial BRM, forms a southeasterly thickening wedge that is progressively truncated by erosion at the Saalian Unconformity towards the NE over the crest

SCHOONER FIELD 815

Fig. 5. Structural map of the Schooner Field based on pre-stack depth migration interpretation of the 1997 3D seismic survey. Note the more complex fault pattern compared with Figure 3.

of the structure penetrated by the 44/26-4 well (Fig. 5). The main reservoir consists of the fluvial BRM, containing more than 98% of the reserves, with the remainder in the fluvio-deltaic CM.

Top seal is provided by the thick Silverpit Formation evaporites and shales that overlie the Saalian Unconformity.

Stratigraphy

The stratigraphical succession (Fig. 2) in the Schooner Field area can be summarized as follows:

Carboniferous. The oldest sediments drilled in the Schooner Field belong to the Namurian age fluvio-deltaic Millstone Grit Group, as encountered in the 44/26-4 well (at least 324 ft). These are overlain by a 2900 ft thick fluvio-deltaic and fluvial Westphalian succession that can be subdivided into the CM and BRM. The transition from Westphalian A to late Westphalian B interval shows a gradual decrease in channel size and sand content (down to 5-10% net-to- gross) with a corresponding decline in reservoir potential. From the late Westphalian B onwards, a gradual increase in sand content is recorded into the Westphalian C which is represented here by Upper Coal Measures (21% net-to-gross) and lower Barren Red Measures Group (28-38% net-to-gross). On seismic sections, the transition from CM into BRM is usually characterized by the increased trans- parency of the seismic and disappearance of continuous clear seismic reflectors representing the coal bearing intervals in the CM. The main reservoir belongs to the Westphalian C intervals.

Data collected by Shell/ExxonMobil during the last ten years suggest that the standard lithostratigraphic scheme (Cameron 1993) does not accurately reflect the relationship of units in the Upper Carboniferous in the Silverpit Basin. Sedimentological, chemostrati- graphical, and biostratigraphical investigations indicate that a clear change in depositional environment and therefore reservoir charac- teristics exists between the formerly defined (Cameron 1993) 'Lower Schooner Formation' (i.e. Coal Measures) and the 'Lower and Upper Ketch Members' (i.e. lower and upper BRM). Moreover, there is evidence of an important erosional event (unconformity) at the base of the Barren Red Measures Group. The lower and upper BRM intervals contrast both in reservoir quality (good in the lower BRM, none in the upper BRM), and provenance. Heavy mineral analyses and zircon age dating (Morton et al. 2001) suggest that the north-northeastern provenance of the lower BRM interval strongly contrasts to the south-southeastern (Brittany?) source area of the upper BRM which can be related to the onshore Halesowen Formation (Westphalian D, English Midlands; Glover et al. 1996; Besly 1998). Palynological analysis in the Silverpit area (McLean 2000) also provide new evidence of a late Westphalian C age for the base of the BRM. Therefore, because of both the contrasting litho- logical, sedimentological and mineralogical characters between the lower and upper intervals within the BRM and the different depo- sitional environment between the BRM and the underlaying CM, the new Shell's Southern North Sea stratigraphical nomenclature proposes to distinguish the Coal Measures Group from the Barren Red Measures Group. Within the BRM Group, the 'Lower and Upper Ketch Members' (Cameron 1993) are promoted to the Formation rank and named Ketch Formation and Boulton Forma- tion, respectively. The former takes the name from the Ketch Field (wells 44/28-1 and 44/28-2) whereas the latter takes the name from the Boulton Field (well 44/21-3) where the fluvio-lacustrine facies of the upper BRM are well represented.

The Coal Measures Group is in turn subdivided in three formations. These are, from bottom upwards: the Caister Forma- tion (Westphalian A), the Westoe Formation (Westphalian B) and the Cleaver Formation (late Westphalian B-early and middle West- phalian C). The terms Schooner Formation, Middle Coal Measures and Lower Coal Measures are thus abandoned (Fig. 2B).

The variable thickness of the Westphalian succession is pri- marily controlled by the Saalian Unconformity, which progres- sively erodes the Carboniferous succession towards the NE. In the Schooner Field, only the sand-rich Lower Ketch Formation is present. The Coal Measures Group (CM) is only fully penetrated by the 44/26-4 well where it has a thickness of 2900 ft. The measured BRM thickness ranges between 0 and 915 ft depending on the depth reached by the erosional Saalian Unconformity.

Sand distribution within the Carboniferous varies vertically probably due to both tectonic and climatically driven basin evo- lution and subsequent change in sedimentation style (Besly 1987; Stone & Moscariello 1999).

Fig. 6. Tectonic setting of the Silver Pit Basin indicating the Schooner Field location. Note the Dogger shelf in the NE that could represent the possible source area of the fluvial system during the Upper Carboniferous time period.

Permian. The lower Permian is represented by the Silverpit For- mation (Rotliegend Group), which developed in a desert lake as interbedded evaporites and claystones.

816 A. MOSCARIELLO

This is overlain by the Zechstein Group, which in this area dis- plays a variable thickness ranging between 3400 and 5650 ft forming a major elongate salt swell overlying the field. This group includes halites, anhydrite and carbonates. Extensive movement of the salt, coupled with faulting has contributed to the deformation and dis- placement within the salt of mid-Zechstein couplets of anhydrite and carbonates (i.e. Haupt Anhydrite and Platten Dolomite). These intervals, known as Zechstein 'rafts', form a high acoustic contrast within the salt attenuating and hence disturbing the seismic imaging of the underlying horizons. The rafts, being vertically displaced, are potentially over-pressured and represent a drilling hazard.

Triassic. At the base of the Triassic is the Bacton Group, which consists of about 1500 ft thick succession of reddish-brown flood- plain and lacustrine mudstones and fluvial sandstone (Bunter Shale and Bunter Sandstone Formations). The Bacton Group is overlain by the Haisborough Group, represented by marine and subordinate lacustrine evaporites, mudstones and limestones. The Upper Trias- sic is absent having been eroded during the Lower Cretaceous uplift (Cimmerian Unconformity).

The 1997 3D seismic processing has resulted in a revised fault pattern (Fig. 5) where faults have been classified according to orientation, age and throw magnitude. Fault compart- mentalization, related to subseismic faulting, is now thought to play a big role in explaining well decline and recovery factors that have not met expectations (see below). Analogue studies (Knipe 1999) suggest that various fault sealing mech- anisms (i.e. juxtaposition, clay smear, cataclasis, development of phyllosilicate framework) are acting in this type of reser- voir, and sealing potential during field production time is strongly controlled by the reactivation history of each fault. However, the featureless nature of the BRM and the low seis- mic resolution (25 m at best) make it difficult to directly image the reservoir sand/shale distribution and no direct imaging of lithofacies juxtaposition is possible.

Reservoir

Coal Measures Group

Jurassic. The entire Jurassic succession is also missing from the Schooner Field area having been eroded by the Lower Cretaceous age, Cimmerian Unconformity.

Cretaceous. The uppermost Lower Cretaceous is represented by the argillaceous Cromer Knoll Group, which is overlain by the Chalk Group (Upper Cretaceous) consisting of a thick sequence of recrystallized and chert-rich limestones, chalks and marls. This is locally affected by the Oligocene (Pyrenean) Unconformity.

Tertiary-Quaternary. The Tertiary is represented by the 225ft thick North Sea Group, which consists of marine and glacio-marine unconsolidated argillaceous sand, clay and silt.

Trap

Trap type and seals

As for many of the fields in the Silver Pit Basin, the Schooner Field trap is a complex elongate NW-SE-trending anticlinal closure, formed by a succession of movements (Cretaceous and Tertiary tectonic inversion) along Hercynian trends. Top seal at the Saalian Unconformity level is provided by the thick Silverpit Formation (Rotliegend Group) consisting of desert-lake shales and evaporites.

Faults

The Schooner Field is characterized by two very distinct fault generations:

(1) A series of major NW-SE trending faults with considerable throw. Some of these only show displacement within the Car- boniferous, whereas others clearly show displacement all the way up to the Zechstein evaporites, probably due to reactiva- tion at a later stage. Of particular importance is the major NNW-SSE high-angle transpressional oblique-slip fault system, which delineates the northern flank of the steeply dip- ping block on the SE of the Schooner Field (Figs 5 & 6).

(2) A series of NE-SW trending steep faults, which only affect the Carboniferous section. As the BRM is seismically transparent, minor faults are not visible within this zone. However, numerous minor faults are visible in the underlying CM.

The Coal Measures Group represents 30% of bulk rock volume of the reservoir (2% of reserves), as only a short sequence of the Middle and Upper Coal Measures (Westphalian B-C) is present above the free water level (FWL) (Fig. 3). As a result of the thin isochore (maximum thickness measured above FWL is approxi- mately 395 ft in 44/26-2 well), the CM has been modelled as a single lithostratigraphic unit (Fig. 7), although biostratigraphic markers (McLean 1995) can be used for further reservoir subdivision. The Middle Coal Measures is partly penetrated by all the wells in the NW part of the field where the BRM is thin or absent.

Sedimentary facies types. The penetrated Coal Measures are char- acterized by a laterally variable low net/gross ratio distribution ranging between 19 and 22%. The following sedimentary facies have been recognized:

Composite low-sinuosity channel fills. These consist of 10-30 ft (3-9 m) thick vertical stacks of 5-15 ft (1.5-4.5 m) thick sand bodies. Typically, these show a vertical grain-size distribution ranging from medium to fine sand. Primary sedimentary structures consist of trough cross-bedded and ripple-laminated sandstones suggesting deposition in low-energy river channels characterized by peri- odic surges as indicated by the numerous reactivation surfaces. A low, blocky gamma ray (GR) response and clear density/neutron (FDC/CNL) log positive separation characterize this facies.

Single low-to-high sinuosity channelfills. These consist of 5-10 ft (1.5-3 m) thick sand bodies formed by fining upward successions of trough cross-bedded and ripple laminated fine to medium sand- stone, which are frequently capped by coals or coaly shales. This sediment association suggests deposition in a low energy fluvial environment developed on a very low gradient alluvial plain. Fine- grained deposits associated with this sand bodies suggest deposition in calm environment probably as consequence of channel abandon- ment. The GR response is characterized by a smooth bell shape and again clear FDC/CNL positive separation.

Proximal overbank, crevasse splay deposits. These are formed by 2-5 ft (0.6-1.5 m) thick, medium to fine-grained sandstone showing ripple lamination (e.g. climbing ripples) and shale drapes towards the top indicating sequences of rapid deposition followed by set- tling processes in a temporary flooded interfluvial plain. Bioturba- tion is common. In logs, this facies has an intermediate GR response and FDC/CNL positive separation.

Floodplain deposits. These are represented by distal overbank and lacustrine massive, horizontally or ripple laminated carbonac- eous grey and black shales, interbedded with very fine sandstones and siltstones and coal seams. Bioturbation, sideritic concretions and plant fragments are abundant. This facies usually exhibits a spiky, high GR response. Coal seams are typically characterized by a spiky, low FDC signature and FDC/CNL negative separation.

SCHOONER FIELD 817

Fig. 7. Well correlation based on gamma ray logs and chemostratigraphical analyses throughout the Schooner Field. Note the typical gamma ray log signature of low net-to-gross fluvial reservoir characterized by composite and single channels generally showing blocky and bell shapes respectively. FWL, free water level.

Intervals with highest GR signatures (>200 API), ranging from 3 to 40 ft thick, are interpreted as the results of deposition during marine transgression phases.

Depositional setting. The depositional environment during the CM accumulation is interpreted to be a waterlogged lower coastal plain cross-cut by fluvio-deltaic meandering rivers (Besly et al. 1993). The area was permanently occupied by swamps and brackish- to-freshwater lagoons in which fine-grained sediments and plant material (coal) accumulated. Periodically, marine incursion also occurred, leaving distinct basin-wide shale markers (marine bands). At the top of the succession, the channels have a more braided character (increase in average grain size) probably indicating a shift to a more proximal fluvial style, which is considered as a precursor of the BRM deposition. This would correspond to a rejuvenation of the sediment source areas probably caused by a tectonic uplift of the sediment provenance area (Besly et al. 1993).

Barren R e d M e a s u r e s

Most of the Schooner gas reserves (98%) are contained in the BRM (Lower Ketch Formation), which forms 70% of the gross rock reservoir volume. A reliable stratigraphical subdivision is essential for understanding and modelling facies distribution within this reservoir, and thus for predicting reservoir performance. Strati- graphical subdivision is, however, difficult in the BRM. Due to the intense oxidation of the sediments, traditional biostratigraphical techniques cannot be applied. Therefore, initial stratigraphical sub- division of the reservoir was based on the identification of key gamma ray signatures (corresponding to assumed 'flooding surfaces' related to t emporary lacustrine expansions?) and thus peak cor- relation between wells was performed (Mijnssen 1997). This litho- stratigraphical subdivision divided the reservoir into three units: (1) a 300-400 ft thick basal sand-rich unit (BRM A); (2) a middle shale-rich layer (BRM B), approximately 120ft thick; and (3) an upper sand-rich unit (BRM C), variable in thickness depending on

the level of erosion at the Saalian Unconformity (maximum mea- sured thickness 275 ft).

Within this lithostratigraphical framework, correlation of sand bodies based on GR responses was then performed between wells (with well spacing 800-2200 metres). The marked variations in channel distribution from the A and C units to the B unit were interpreted, using sequence stratigraphy criteria, as being the result of changes in relative base level (Mijnssen 1997).

However, two years of production history did not match the pre- dictions of the existing static and dynamic reservoir models (Stone & Moscariello 1999). Initial inflow rates (average of 45 MMSCF/D, range of 12-80 MMSCF/D) matched reasonably well with predicted rates (45-50 MMSCF/D), but total connected well reserves from early material balance data did not match model forecasts and well decline rates were also much larger than initially predicted. This suggested that the sands were less well connected than assumed in the original reservoir model.

Recent analogue data from the Green River Formation of the Uinta Basin, Utah (Keighley et al. 1998, 1999) suggested that sand body width/thickness ratio and lateral connectivity was over esti- mated and that significant lateral variability in the net sand distri- bution could be expected over hundreds of metres.

The original modelling assumptions were therefore revisited (Stone & Moscariello 1999). The previously assumed channel geometry parameters were revised to obtain a more realistic static model (e.g. the maximum channel width was changed from 4000 m to 1800 m).

To re-evaluate the internal stratigraphical zonation of the BRM, a chemostratigraphical correlation technique was chosen to generate a robust stratigraphical framework (Pearce et al. 1999; Stone & Moscariello 1999). Vertical distribution of chemical ele- ments and their relative abundance were analysed for trends and rapid shifts. Pattern matching between wells was used as the basis for correlation. A five-zone subdivision of the BRM (Fig. 7) was constructed based on correlatable geochemical signatures in eight wells. This subdivision has been interpreted as the response to climatically driven changes in weathering cycles in the catchment area and floodplain groundwater conditions. Petrographical data

818 A. MOSCARIELLO

lacustrine and polygenic composite palaeosols swamp deposits (Pedofacies 2 to 4)

Fig. 8. Core photograph of a typical sedimentary interval in the Barren Red Measures. Massive and fining upwards fine grained sandstones are interbedded with fine-grained floodplain deposits showing palaesols with different degrees of maturity.

did not show any considerable change in sediment provenance during the deposition of the BRM.

Sedimentary facies types. The BRM part of the Schooner reser- voir is characterized by a low to moderate net/gross reservoir ratio (30% mode) and a high degree of internal, lateral and vertical

reservoir variability (Figs 8 & 9). Based on the classification initi- ally proposed by Mijnssen (1997), the BRM facies can be described as follows:

Composite low-sinuosity channel fill. This consist of 12-30ft (4.5-9m) thick vertical stacks of 2-Sft (0.6-2.5m) thick sand bodies characterized by several lithologies: poorly stratified, clast- supported, conglomerates consisting of poorly sorted, sub-angular, fine to medium pebbles and granules; trough cross-bedded sand- stones and ripple-laminated medium to coarse sandstones. Often, numerous reactivation surfaces are present. Generally, no obvious grain-size trends are present within the channel fill although sand bodies are often capped by parallel laminated sands and silts (Fig. 9). The sediment composition and sedimentary features of these chan- nel fills suggest deposition in a fluvial environment dominated by competent flows associated with high energy flood events. Massive conglomerate and coarse sand with trough cross-bedding at the base of the channel fill (Fig. 9) are interpreted as the result of migration of large scale bedforms developed in braided stream channel. A blocky GR response (Fig. 9) and a clear FDC/CNL positive separation characterize this facies.

Single low-sinuosity channel fill. This genetic unit consists of 8-15 ft (2.5-5 m) thick medium to coarse sandstone packages char- acterized by trough cross-bedding and ripple-lamination. Usually, this facies shows a fining-upwards sequence resulting in a bell shaped GR response and clear FDC/CNL positive separation.

Based on log properties both composite and single channel sediments have been assigned to type I or type II, the former having lower GR and higher log porosity signatures.

Proximal overbank deposits crevasse splay deposits. These are formed by 4 -8 f t (1.2-2.5 m) thick, medium to fine-grained sand- stone. Similarly to the ones described for the Coal Measures inter- val, they are characterized by c. 1-3 ft (30-90 cm) thickfining upward sequences formed by homogeneous, structureless, medium sand at the base passing upwards to ripple lamination (e.g. climbing ripples) and shale drapes at the top. These sequences indicate successive events of rapid deposition followed by settling processes in a tem- porary flooded interfluvial plain. Bioturbation and root mottling characterize this unit. A spiky GR response (Fig. 9) and a vague FDC/CNL positive separation characterize these units.

Floodplain deposits andpalaeosols. These consist of laminated or massive fine-grained sandstones and horizontally laminated mud- stones (Figs 8 & 9) accumulated on a distal floodplain where temporary shallow lacustrine environments could develop. The thickest continuous succession of these sediments reaches 60ft

Fig. 9. Schematic facies assemblages characterising the Barren Red Measures reservoir.

SCHOONER FIELD 819

(20 m). Pedogenetic features (i.e. rootlets, bioturbation, mottling, nodules) are very common (Besly & Turner 1983) indicating the presence of vegetation occupying the floodplain. Four types of pedofacies (Fig. 10) have been distinguished according to the degree of palaeosol maturity (Moscariello 2000; Moscariello et al. 2002). Vertical pedofacies distribution usually show 20-45ft (6-15m) thick regular cycles mostly consisting of overbank deposits showing an upward increase in degree of soil maturity. Each cycle usually starts with a channel fill or overbank deposits which do not display pedogenetic features. These typically directly overlay very mature palaeosols belonging to the previous cycle. The vertical repetition of these trends (Fig. 10) indicate a dynamic fluvial system characterized by periodic channel avulsion over the floodplain where intense pedogenetic processes could take place. Lateral distribution and ver- tical patterns of pedofacies types is used as an indicator of different styles of lateral and vertical aggradation rates. The vertical distri- bution of pedofacies is also consistent with the chemostratigraphi- cal zonation supporting the use of this technique for reservoir subdivision (Stone & Moscariello 1999). High and spiky GR

response characterizes this genetic unit. Sonic shear-waves respond to the geomechanical properties of the fine-grained which have been pedogenically modified (i.e. peds structure, vertical fracture planes) and can thus be used (Moscariello 2000) to infer pedofacies vertical distribution (Fig. 10) and ultimately to reconstruct channel distri- bution within the reservoir.

Depositional setting. The overall depositional setting of the BRM is interpreted to be fluvial, characterized by braided channels drain- ing a low gradient alluvial plain probably developed in an endorheic basin. Within this system, major low-sinuosity channels developed. Minor single channels formed small subsidiaries flowing between the large channels. Proximal overbank deposits formed adjacent to the main channel areas during flooding events while in the large interfluves only fine-grained deposits were accumulated allowing the development of vegetated soils. Log correlation and isopach map- ping indicate that the channels are predominately oriented NE to SW. Moreover, provenance studies using Sm-Nd isotope anal- yses indicate a source area dominated by Palaeozoic igneous rock

44/26 a -A 1 z Core

GR (API) Pedofacies Thickness (ft)

50 150 250 12800

12900 "

13000

1310o J

"1- 13200 4

r- 13300 a

!3._4~ , - - " ' - 3 . . . . .

13500 / /

13800

I I 13900 . . . . . . . . . . . . . . . . . . . . . . . . . . . L|

un i ts ~IL,~I~L~

(

44/26-3 Core

GR (API) Pedofacies Thickness (ft) 1 2 3 4 0 10 20 0 50 .100 1 2 3 4 0 10 20

12800 -

5 12900 "

5 -

4 13100

A

~ - - - " Z

Pedofac ies cyc lo -s t ra t ig raphy

GR (API)

0 100 200

13500-

13600-

13700"~

13800-

13900

Pedofac ies Thickness (ft)

1 2 3 4 0 10 20

Reservo i r archi tecture based on pedofac ies

vert ical and lateral d istr ibut ion

44126-3

44/26a-A1z ? ~ .........

~ - - - - - N n i t s 4 - 5 !

44/26a-A1z 44/26-3

D m D channel palaeosol f loodplain

Pedofacies lateral distribution suggests an active avulsing fluvial system during deposition of Units 1-2 and 3. Channel are randomly distributed within floodplain deposits and potentially well connected. On the other hand, Unit 4 and 5 formed during a more stable system where entrenched channel belt developed.

Pedofacies vertical distribution is characterised by high to low aggradation cycles (from Pedofacies 1 to 4). These cycles are typically of a frequency of 4-6 per chemostratigraphic unit.The thickness of each pedofacies type varies vertically, generally showing thinning upwards successions whose base and top correspond to the boundaries of the chemostratigraphical units.

Fig. l& Example of lateral correlation and pedofacies distribution for two wells in the Schooner Field (Upper Ketch Formation, Westphalian C/D). Occurrence and thickness of four types of pedofacies recognized in core are plotted against gamma ray log.

820 A. MOSCARIELLO

Fig. 11. Sm-Nd isotope composition of Barren Red Measures sandstone indicating that a possible source rock could be represented by Palaeozoic igneous rock and Proterozoic gneiss similar in character to the basement of the Norwegian Domain.

and with subsidiary Proterozoic gneiss similar in character to the basement of the Norwegian Domain (Fig. 11). This data suggests a source area located to the N - N E of the Silver Pit area, to the east of the Mid North Sea High where plutonic rocks occur (Dogger Shelf, Fig. 6).

In detail, the data suggest that two main chemostratigraphical unit assemblages, developed under different sedimentary basinal settings, can be identified on the basis of similarities of net-to-gross, pedofacies, and reservoir properties (porosity, permeability) distri- bution (Stone & Moscariello 1999). The lower three chemostrati- graphical units (1 to 3) can be distinguished from the upper two units (4 and 5) by different internal geometry. This geometry is believed to be directly controlled by the variation of several factors

over time. These are: (1) climatically driven sediment supply to the alluvial plain; (2) climatically controlled fi'equency of catastrophic flood events, and in turn channel avulsion; and (3) the modifica- tions in tectonic regime, which induced changes on alluvial plain evolution and channel distribution.

During deposition of Units 1, 2 and 3 a strong and prolonged subsidence during the Late Carboniferous resulted in large amounts of accomodation space occupied by the fluvial plain aggradation. Fluctuation in base level resulted in an alternation of braided river systems formed during relative base level (lacustrine) low stands and meandering river systems, formed during high stands (Besly et al. 1993). During this period, the braided river system constantly avulsed and bifurcated, resulting in a wide range of channel sizes and distribution, the latter being controlled by autocyclic processes related to climate-driven discharge into the basin.

During depositions of Units 4 and 5 however, the fluvial chan- nels are temporarily confined in specific areas, forming stacked channel belts up to 6 -10m (20-30 ft) thick. This is likely to be asso- ciated with longer time scale, local (lacustrine ?) relative base level falls which induced minor, short lived incisions, which in turn favoured the formation of composite stacked channels. The changes in the fan topography and overall evolution of the sedimentary basin most likely resulted from a combination of climatic factors (e.g. progressive increase in aridity at the end of the Westphalian D (Besly 1987, 1990) and increase in tectonic activity (subsidence rate, tilting) related to the early Variscan orogenesis (Leeder & Hard- man 1990).

P o r e types and diagenesis

The petrographical composition of the red beds forming the Schooner reservoir rock is the result of early diagenetic and burial- related processes. Both oxidation and reduction processes acted throughout the early post-depositional and burial time inducing a complicated petrographical assemblage. Early diagenetic assem- blages mainly consist of Fe-oxides, authigenic quartz, kaolinite, illite and pyrite. Siderite, heamatite and ankerite commonly form pore-filling cement (Besly et al. 1993). In the BRM sequence, petrographic analyses indicates that porosity is related to feldspar grain dissolution. Kaolinite and illite are associated with partially dissolved feldspars or oversized pores that represent the sites of former feldspar grains. Porous reservoir generation then post dates the development of reddening and is thought to have taken place during deep Mesozoic burial (Besly et al. 1993). Elevated tem- peratures and aggressive (low pH) formation waters suggested as

Fig. 12. Porosity-permeability crossplot showing the distribution between different facies. The plot shows both core and log-derived data.

SCHOONER FIELD

Table 1. Reservoir property distribution for each BRM chemostratigraphic unit and CM

821

Reservoir unit Maximum porosity Mean porosity Maximum permeability (%) (%) (mD)

Mean permeability (mD)

Mean net-to-gross

BRM 5 17.8 10.8 1990 116 0.30 BRM 4 16.4 9.3 954 70.8 0.28 BRM 3 18.4 12.6 2100 172.1 0.35 BRM 2 17.9 8.27 520 90.1 0.33 BRM t 18.9 11.6 1895 193.4 0.38 CM 20.1 6.2 1000 33.4 0.21

being associated with maturation of organic matter, are the possible cause of this enhanced porosity generation (Cowan 1989).

Porosi ty and Permeabi l i ty

Reservoir quality in the sandstones of the CM and BRM is gen- erally good to excellent, with an average porosity of 12% and a wide range of permeabilities from 10 to 2100mD. Core analysis indicates that in general the thicker sands, which have the most significant contribution to the total gas volumes, have good reser- voir properties. Typically, channel fill facies have mean porosity ranging between 11 and 13% while proximal overbank facies have between 4 and 7%. Core permeability for channel fill ranges between 1 and 1000 mD (air permeability) with an average of about 10 mD. Core permeability are consistent with log-derived permeability cal- culated using multivariate functions using porosity, gamma ray, volume of shale and calibration to core permeability. Data from nuclear magnetic resonance (NMR) analyses are consistent with the computed permeabilities from logs. For proximal overbank deposits, mean permeability values measured in core and log are also com- parable (average of 0.01 mD). Porosity and permeability distribution per facies are shown in Figure 12.

The data from each individual interval showed considerable variation in porosity and even more so in permeability. This is because, within the channel sand bodies, there are a variety of sub- facies and grain sizes. Typically, the fine-grained upper parts of the sand body exhibit lower permeabilities than the coarse-grained inter- vals near the channel bases. The sandstones are, however, embedded in impermeable floodplain mudstones that comprise 65-70% of the stratigraphical section. Consequently, significant concern exists about sand body connectivity and the impact it will have on the recovery of gas.

As porosity-permeability characteristics do not vary greatly from unit to unit (Table l), gas inflow performance is primarily controlled by the number of channel sands the wellbore penetrates, and the degree of lateral connectivity of these channels.

Pressure relationships

Formation pressures obtained with repeated formation test (RFT) logging tools in the 44/26-2, 44/26-3 and 44/26-4 wells are plotted in Figure 13. All the Schooner Field wells are on the same water and gas line. Formation multi tester (FMT) pressures taken in the BRM sequence in the 44/26-2 well indicate a gas gradient of 0.12 psi/ft.

The free water level (FWL) is estimated to be 13 075 ft TVDss (3985 m TVDss). This figure is also indicated by saturations from capillary pressure curves and from resistivity data. To date, despite the high probability of fault compartmentalization, as could suggest the rapid decline of a couple of wells, no indication for different FWLs over the field have been observed.

Fig. 13. Pressure plot for the Schooner Field based on exploration wells 44/26-2, 44/26-3 and 44/26-4.

Source

The source of the gas in the Schooner and Ketch fields are the Namurian and Westphalian coals. The source has two components, carbonaceous shales (c. 1% TOC) and coals (c. 60% TOC). The types of kerogen are II/III-III. The potential yield for the shale is 0.14 MCF/acre-ft and 7.0 MCF/acre-ft for the coal (Cornford 1986). Measured and estimated Vitrinite Reflectance maturities range between 0.8 and 1.1 (%Ro) at the level of the Saalian Unconformity, implying a very extensive gas kitchen at depth within the Carbon- iferous succession. The current burial depth, the gas kitchen and hydrocarbon charge should still be active, below depth of about 13 000 ft TVDss (3.5 km TVDss).

Over much of the area, the presence of the Silverpit Formation and a thick Zechstein salt succession precludes hydrocarbon migra- tion from the Coal Measures into the upper reservoirs such as the Triassic Bunter Sandstone.

Migration paths are supplied by the sandstones within the Westphalian BRM and CM, which have extensive areas of contact with both the coals and carbonaceous shales. Source and reservoir sandstone thus lie at the same stratigraphical level.

Migration timing coincided with the time of maximum depth of burial during the Jurassic and Cretaceous. The gas first migrated to the structurally higher flanks of the Sole Pit Basin. Later, during and after the structural inversion and formation of the trap in the Late Cretaceous, the gas re-migrated back into the field. The fact

822 A. MOSCARIELLO

Fig. 14. Graph summarising the variation over time of gas initially-in-place (GIIP) and reserve estimates. The most recent estimate was derived from the latest modelling exercise based on re-examination of geological, petrophysical and production data. UR, ultimate recovery; GRV, gross rock volume.

that the gas remained trapped until the present demonstrates the efficiency of the seal formed by the Silverpit Claystone.

Reserves and production

Gas-in-place

The Schooner Field maximum gas column is 1275ft (388 m) with the crest at 11 800 ft TVDss (3596 m TVDss). The BRM contains 88% of the gas-in-place, with the remaining 12% being located in the CM. The Schooner Field expected gas initially-in-place (GIIP) is currently estimated at 29.98 Gm 3 (1059 BCF). Changes in esti- mated GIIP and ultimate recovery (UR) calculation over the time are summarized in Figure 14. Considerable variations in gross rock volume (GRV) and gas volume estimates resulted from the 1989 first 3D seismic interpretation. A further change in GRV resulted from the 1997 PreSDM interpretation, with an increase of about 35% since publication of the field development plan.

The average expected condensate/gas ratio (CGR) over field life is 12 BBL/MMSCF and expectation natural gas liquid (NGL) re- coverable is 1.5 MMBBL for the entire field. The reservoir fluid is a wet gas with no in-situ liquids at original conditions. The produced BRM gas stream composition is expected to have about 1.15 mole% of carbon dioxide, 4.18 mole% of nitrogen and CGR at Theddle- thorpe (Fig. 1) process conditions of 14.6 STB/MMSCF. No data presently exists for the CM gas. The gas composition is illustrated in Table 2.

Recovery and connectivity factor

The recovery mechanism in the Schooner Field is natural deple- tion. The UR is dictated by two main parameters: (1) the sand volumes connected to the development wells, and (2) abandonment reservoir pressures. Major uncertainties in the geological model and, most importantly, reservoir connectivity make it difficult to provide accurate estimates of recovery factors. The reservoir abandonment pressures are dictated by the Transportation & Processing (T&P) agreement with the Caister-Murdoch System (CMS) owners. On the CMS route compression will be needed when tubing head pressure (THP) falls below 2000 psia. The abandonment THP is estimated to be 300 psia.

Static modelling, based on improved understanding of reservoir complexity and dynamic simulation matching of these models to early production data, resulted in a reassessment of the internal connectivity factor. This is defined as the ratio between the volume of sand connected to the wells and the total volume of net sand in the reservoir. The BRM are the main reservoirs accounting for

more than 98% of the gas reserves in Schooner Field. The most likely recovery factor for the BRM is 63.5% while for the CM is only 8.7%. Based on these figures, dynamic reservoir simulation indicates that the overall Schooner Field recovery factor is approxi- mately 58%. The expectation U R (wet) for the current 10 well pene- tration development concept is predicted to be approximately 17.34 Gm 3 (612 BCF).

Table 2. Gas composition from the Schooner Field

Component Recombined gas (Mol%)

Methane 83.72 Ethane 6.24 Propane 1.89 Iso-butane 0.35 N-butane 0.41 Iso-pentane 0.15 N-pentane 0.14 Hexanes 0.33 Heptanes plus 0.48 Nitrogen 4.51 Carbon Dioxide 1.16 H20 0.62

Total 100.00

7000

S c h o o n e r F i e l d

P / Z v . F i e l d C u m u l a t i v e P r o d u c t i o n

6 o o o . . . . . . . . . i i

5000 ".--.-..A..r : ..................... tl N 4 0 0 0 - ' .................................. ~ ...................... - - . ~ : . . . . .

3 0 0 0 i [ ....... i E ' | i ~ " I 2000 . . . . . . . ~,,~ . . . . . . i~, . . . . . . . . . . . . . . . . . . . . . ~' J - - ._~. - . ....

1 0 0 0 i , I ......................... ! ............. + . . . . . . . . . . . . . . . . . . . .

o J ! , , I J

0 20 40 60 80 100 120 140 160

C u m u l a t i v e P r o d u c t i o n ( B C F )

Fig. 15. P/Z v. cumulative production for the first Schooner development wells. P, reservoir pressure; Z, real gas factor, which varies between 0.95 and 1.2 depending on reservoir pressure.

SCHOONER FIELD 823

Schooner Field Historical Gas Production to lstApri l 1999

200 T

180 4̀ 6" r 160

140 U

120

loo

I1r 80

(,,9 60

0

120 1.4

- i ~ ' 8 1.2 loo o ~

m z m

8 o ~ ~ 1

~= ~ 0.8 t~ 60 �9

.> .~ ~ 0.6 40 -~

E "~ 0.4 �9 -I O

U U

20 0.2

Schooner Field Historical Condensate Production to 1st April 1999

0 0

1997 1998 1999 1 uue 1997 1998 1999

Y e a r s Y e a r s

0.8 ~,

0.7

0.6

0.5 e-

0.4 o U

0.3 ~_

0.2

0.1 U

0

Fig. 16. Production history graph showing the decline curve which is typical of wells in this type of low net-to-gross, moderately to poorly connected reservoirs.

Production rate

Gas production began on 1st October 1996 from four wells. To date, seven development wells have been drilled into the Schooner Field reservoir. The first development phase was forecast to start at a peak plateau rate of 130 MMSCF/D from eight wells. However, the cur- rent average flow rate is 90 MMSCF/D from 7 wells. After the first three years of production, a marked pressure decline was observed that did not match expectations (Fig. 15). The initial productivity of the wells is some 20% lower than expected based on appraisal well tests and simulation. As a result of the current field performance, a reassessment of the reservoir model is in progress in order to include more realistic representation of the internal reservoir architecture (both structure and channel geometry) and rock properties.

in the CM and BRM also offer potentially high volumes of GIIP but with a low recovery factor. The use of new recovery technologies, such as multilateral wells and under balanced drilling, could unlock these resources.

The work described in this paper is based on a number of studies performed by employees of Shell U.K. Exploration and Production, Esso Exploration and Production UK Ltd and various contractors. I am particularly grateful to my colleagues M. Alberts, W. Epping, D. Grant, E. Legius, M. Klingbeil and I. Reid of the Silver Pit Subsurface Team, Southern Gas Supply Group, Lowestoft of Shell U.K. Exploration and Production for their helpful sup- port during the collection of the data. Constructive comments by Bernard Besly, Duncan Mcgregor and Colin North, which improved the quality the manuscript, are gratefully acknowledged. Shell U.K. Exploration and Production, ExxonMobil International Ltd, Cal Energy and TXU Europe Upstream Ltd. are thanked for permission to publish this paper.

Cumulative production

To date, the Schooner Field has produced 3.9 Gm 3 (137.77 BCF) that corresponds to 22% of its initial base case reserves of 17.34 Gm 3.

The field cumulative production data are summarized in Table 3 and Figure 16. The estimated life time production profile since the first development well was drilled in 1996, is shown in Figure 17.

Concluding remarks

The effective development of the Schooner Field depends primarily on the proper understanding of the geological complexity of the low net-to-gross, BRM fluvial reservoir, which contains 98% of the reserves. A re-examination of geological, petrophysical and produc- tion data, integrated with the results from chemostratigraphical, sedimentological and analogue studies, led to the rebuilding of the reservoir model. The new model is felt to be more realistic and reliable for long term production forecasting. However, managing the subsurface uncertainties of reservoir connectivity and structural definition (sub-seismic compartmentalisation), and improving the recovery factor by identifying infill well locations, continue to be the major challenges in this type of reservoir, driving the uptake of new technologies. The lower net-to-gross and tighter reservoir intervals

Table 3. Cumulative production at December 1999

NGL (MMBBL) N/ASS. Wet Gas (BSCF)

During 1999 0.74 41.7 Cumulative 1.59 137.77

Estimates of Wet Gas Reserves are updated with cumulative production.

Schooner Field data summary

Trap Type dip closure Depth to crest 11 800 ft Lowest closing contour 13 075 ft GWC 13 075 ft OWC Gas column 1275 ft Oil column

Pay z o n e

Formation Barren Red Measures and Coal Measures

Schooner Field - Lifetime Production Profile

140

t,,t) 120

o'~ "o 100

o 80 >

< ~ 60

~ 4o e- 20 c

< o

O~ O~ 0 O~ O~ 0

LO O0 v- 0 CD T- O 0 0 C~ C~ C~

�9 ~- ~ O

O O O Cq Cq Cq

Y e a r

Fig. 17. Lifetime production profile of the Schooner Field.

824 A. MOSCARIELLO

Age Westphalian C/D Gross thickness (max) 1275ft Net/gross (range) 20-38% Porosity average (range) 10-13 % Permeability average (range) 30 1000roD Gas saturation average (range) 70-85% Productivity index 0.015 MMSCF/D psi

Petroleum Oil density Oil type Gas gravity 0.66 Viscosity 0.0277 cp Bubble point Dew point 2930 psig Gas-oil ratio Condensate yield 12 to 15 BBL/MMSCF Formation volume factor 0.0036 Gas expansion factor 286.53 SCF/RCF

Formation water Salinity 93 700 NaC1 eq ppm Resistivity 0.027 ohm m

Field characteristics Area 13 590 acres Gross rock volume 5 026 420 acre ft Initial pressure 6475 @ 12 800 ft TVDsspsi Pressure gradient 0.11 @ 6475 psi psi/ft Temperature 230 + 15~ Oil initially-in-place Gas initially-in-place (wet) 1059 BCF Recovery factor 58% Drive mechanism natural depletion Recoverable oil Recoverable gas (dry) 612 BCF Recoverable NGL/condensate 7.41 MMBBL

Production Start-up date October 1st 1996 Production rate plateau oil Production rate plateau gas 130 MCF/D Number/type of well 7 single deviated

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