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The Relationship Between Recovery Efficiency and Depositional Setting in a Deltaic Plain Environment*
By
Robert C. Shoup1
Search and Discovery Article #40240 (2007)
Posted May 23, 2007
*Adapted from oral presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007 and poster presentation at the 2006 AAPG International Conference and Exhibition; November 5-8, 2006; Perth, Australia.
14001 Fannin Street, Houston, Texas 77004 ([email protected])
A Full Field Review was conducted for a structurally and stratigraphically complex field offshore Sarawak. The East portion of the field is a relatively simple, west-plunging flower-structure fold. The West portion of the field consists of a series of normal conjugate faults that formed in response to tensional bending over a deep-seated normal basement fault. These faults result in the severe compartmentalization of the western portion of the field.
There are over 20 separate reservoirs in the field, comprising both channel sands and incised valley fill sequences that were deposited by a generally westward flowing river system. The eastern portion of the field was situated in the upper deltaic plain where deposition was from a fluvial environment, whereas the depositional setting for the western portion of the field was the lower deltaic plain estuarine setting.
Production from the fluvial reservoirs in the eastern portion of the field exhibit little to no aquifer support and recovery efficiencies range from 20 to 35%. Production from the estuarine reservoirs in the western portion of the field have significant aquifer support, and recovery efficiencies range from 35 to 50%.
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A full-field review was conducted for the Temana Field located offshore Sarawak. There are over one hundred separate reservoir compartments in the field. In the course of this study, it was observed that fluvial-dominated reservoirs in the western portion of the field have strong aquifer support and high recovery efficiencies, whereas in the eastern portion of the field, the same reservoir systems exhibit virtually no aquifer support and low recovery efficiencies. This paper focuses on the causes of that observed difference. The Temana Field is situated in the south central region of the Balingian Province of the Sarawak Basin. It is located offshore Sarawak (Figure 1), 35 km west of Bintulu in 96ft (30m) of water (Figure 2). Structural development of the Sarawak Basin initiated during the Cretaceous with subduction and accretionary folding. Eocene aged carbonates were subsequently deposited on paleo-Cretaceous highs and accretionary folds. Clastic deposition followed as sediments shed off of the Rajang orogenic belt south and east of Temana Field prograded into the Sarawak Basin during the Oligocene and Miocene.
The Temana field encompasses two structural regimes, an extensional regime comprising Temana West and a wrench regime comprising Temana East and Central (Figures 3 and 4). Temana West consists of a series of normal conjugate faults that formed in response to both tensional bending over deep-seated down-to-the-west normal faults that delineate the edge of the Balingian Sub-Basin and late stage uplift and thrust faulting believed to be associated with Mid to Late Miocene reactivation of the West Balingian wrench fault. The Temana East and Central structure is a reverse-fault-bounded, elongated east-west trending, west-plunging anticline formed as a high-angle flower-structure fold associated with the right-lateral West Balingian wrench fault (Figure 2). The south-bounding reverse fault is believed to have initially formed as syndepositional growth faults that were reactivated with left-lateral slip during the Mid to Late Miocene reactivation of the West Balingian wrench fault. Westward plunge of the Temana East anticline sets up the Temana Central Field.
Sediments in the southern portion of the Balingian Province consist of siliclastic sediments of Cycles I to VIII (Oligocene to Recent) overlying the Rajang Group (Figure 5), a tightly folded Late Cretaceous to Late Eocene flysch succession (Madon and Abolins, 1999). The provenance for these siliciclastic sediments was the Rajang Orogenic Belt which trends into onshore Sarawak between Bintulu and Kuching (Madon, 1999). On the western margin of the Balingian Province, the Luconia deltaic complex prograded eastward off of the Penian High, which is separated from the Balingian Province by the West Balingian Line, a right-lateral wrench fault system (Figure 2). The Temana Field reservoirs were deposited by the Tatau-Kemana deltaic complex. This deltaic complex, which is situated along the southeastern margin of the Balingian Province, prograded north-northwestward into the Balingian Basin from Sarawak (Figure 2). Return to top.
Modern Depositional Environment Analogs
The evaluation of the conventional cores
indicates that there were both deltaic and delta plain/mangrove swamp
sediments, which include coals, meandering channels, and incised valley
channel systems, deposited at Temana. The recognition of these
depositional facies in the remaining
Previous workers had placed the
depositional environment in a generalized delta to delta-plain setting,
without differentiating between transgressive or regressive phases. The
Studies of modern deltas have shown that there are three basic geomorphic styles of deltas—river-dominated, wave-dominated, and tidal-dominated (Wright and Coleman, 1973; Coleman and Wright, 1975). The present-day Baram Delta is a classic wave-dominated delta. Since any delta forming in the Bintulu region would be subject to the same wave regime as the Baram, it is safe to assume that the delta there was also wave-dominated. The basal section of a prograding delta is marine shale overlain by outer fringe prodelta shale with occasional interbeds of siltstone and thin-bedded sandstones (Figure 6). This in turn is overlain by interbedded sandstone, siltstone, and shale of the inner fringe (LeBlanc, unpublished Shell Training Manual).
The inner fringe grades upward to the
shoreface, which is comprised of thick-bedded, massive mouth-bar
sandstones with thin interbedded shales. The
Mangrove swamps are generally found in tropical to subtropical coastal environments. They typically consist of a series of anastomosing waterways separated by mangrove covered islands (Figure 8). There are two types of waterways within the swamp--tidally influenced estuarine and meandering fluvial channels, which may also exhibit tidal influence.
Fluvial channels in mangrove swamp environments are typically meandering channels; however, they tend to be less sinuous than meandering rivers in other environments due to the effects of the mangroves. The width of these channels can vary from several dozen to several hundred feet, although the overall width of the meander belt within which the channel meanders can be several miles wide. The depth of the channels will range from a few feet up to about 50 feet.
Mudstones are the most prevalent rock
type within the mangrove swamp environment. Coals are also prevalent,
but patchy in their overall distribution within the swamp (Figure
8). Point bar deposits associated with the meandering channel are
the predominant reservoir facies (Figure 9).
During periods of flooding, overbank deposits consisting of laminated
sands and shales are deposited along the channel margins and within the
estuarine waterways. These laminated sections are often characterized by
both low resistivity and low gamma ray contrast in
With the exception of major river
systems, the majority of rivers in the world today have channels less
than 50ft deep; therefore, channel deposits thicker than 50 feet are
most likely associated with incised valley sequences. The facies
distribution of an incised valley
The I60 and underlying I65 reservoirs
were deposited as a series of incised-valley-fill sequences (Figure
11). The I60 incised valley
Fault Block 10/11 is the principal producing block in western portion of the field; having produced over 10.7 MMSTB. There are eight penetrations in this block (Figure 13) of which all but two are completed in the interval.
The initial correlations for this fault
block were that the two thick sands observed were the I60 and I65 sands
(Figure 14). Subsequent to that
interpretation, the TE 41st was drilled and found a different oil-water
contact than that previously observed. To account for that difference,
the original interpreters added a fault. However, that fault cannot be
observed on seismic. An alternative interpretation is that the observed
sands represent 3 ‘shingled’ channels. With this interpretation, the top
sand correlates to the I40, the middle sand to I60 and the basal sand as
I65 (Figure 15). Reservoir characteristics
are given in Figure 16, along with a
representative
The oil-water contact for the I60 sand is
observed in the TE31st1
All three sands have exhibited water
movement through time. In the I40 sand, the TE 26
No gas cap has been observed nor has there been a significant increase in gas production.
Net pay maps were constructed for the I40
sand (Figure 18) and for the I60 – I65 sands
(Figure 19). The STOIIP for the combined
sands ranges from 16.2 to 29.6 MMSTB, with a base case volume of 21.4
MMSTB. The estimated ultimate recovery for the block ranges from 11 to
13 MMSTB assuming a recovery factor of 45 to 55 percent, which is
comparable to that derived from material balance and decline curve
Fault Block 54/99 is the fault block that comprises the central portion of the Temana Field. This is one of the principal producing blocks of the field, having produced just over 20 MMSTB. There are eighteen penetrations in the block, ten of which are completed in the I60 (Figure 20).
The I60 incised valley
Based on limited
The oil-water contact is not observed in
this compartment. RFT
Four wells have had associated water
production--the TE54st, TE56st, TE 70 (horizontal), and TE 71st. The
TE54st and TE56st produced water from the I62 sand as opposed to the I60
sand. The TE70 It is therefore concluded that none of the observed water production is associated with movement of the I60 water level.
There is no evidence for an original gas
cap. Shortly after initial production, the reservoir pressure dropped
below the bubble-point, and a secondary gas cap developed. The gas cap
has expanded as far downdip as the TE 64
The net pay maps were contoured using an incised valley fill model (Figure 26). The accumulation is trapped by the incised valley margin to the north and the south. Deterministic assessment of the STOIIP based on the net pay maps results in a range of 62.7 to 112.1 MMSTB, with a base case volume of 87.6 MMSTB.
Fault Block 10/11 is situated in the western portion of the field, which, at I60 time, was characterized by an estuarine setting where extensive laminated sandstones were deposited in an overbank setting. These laminated sandstones are connected to the channel sandstones reservoirs and provide aquifer support. As a result, recovery efficiencies for Fault Block 10/11 approach 50%. Fault Block 54/99 is situated in the eastern portion of the field, which, at I60 time, was inland of the estuarine setting. As such, there are no connected overbank deposits and therefore, no aquifer support, resulting in a recovery efficiency of 30%. Interpreted depositional settings for the two blocks are illustrated in Figure 27.
Hampson, Gary J., John A.
Howell, and Stephen S. Flint, 1999, A sedimentological and Karlo, John F., and Robert C. Shoup, (2000), Classification of syndepositional systems and tectonic provinces of the Northern Gulf of Mexico: AAPG Search and Discovery, Search and Discovery Article #30004, http://www.searchanddiscovery.com/documents/karlo/index.htm. Madon, Mazlan B. Hj., 1999, Geological setting of Sarawak (Chapter 12) in The Petroleum Geology and Resources of Malaysia, Mansor, M.I. ed.: Petroliam Nasional Berhad (Petronas), Kuala Lumpur, Malaysia. Madon, Mazlan B. Hj., and Peter Abolins, 1999, Balingian Province (Chapter 14) in The Petroleum Geology and Resources of Malaysia, Mansor, M.I. ed.: Petroliam Nasional Berhad (Petronas), Kuala Lumpur, Malaysia.
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Sarawak State Department of Irrigation and Drainage, 2005, 9th and 10th Floor, Wisma Saberkas, Jalan Tun Abang Haji Openg, Kuching Sarawak. http://www.did.sarawak.gov.my/papt/project/maps_htm/bint.html . Swinburn, P., H. Burgisser, and Jamius Yassin, 1994, Hydrocarbon Charge Modeling, Balingian Province, Sarawak Malaysia (abstract): AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, 21 – 24 August, 1994, AAPG Bulletin, v.78, no. 13, p. 62. Tearpock, D.J., and R.E. Bishke, (2003), Applied Subsurface Geologic Mapping with Structural Methods, 2nd Edition, Lawrence G. Walker, ed.: Prentice Hall, New Jersey. |
