Paul D. Crevello

Growth Architecture, Faulting, and Karstification of a Middle Miocene Carbonate Platform, Luconia Province, Offshore Sarawak, Malaysia

The Mega Platform is a 30- 50-km-large and 1.2-km-thick middle Miocene carbonate platform located in the Luconia Province, offshore Sarawak, Borneo. The platform originated in the late early to early middle Miocene on a regional fault-bounded structural high, first aggraded and then backstepped during a series of third-order sea level fluctuations during the middle Miocene (TB2.32.6). The Jintan Platform termination with an area of 8 12 km is one of the prominent backsteps toward the top of the Mega Platform. Three-dimensional (3-D) seismic indicates that growth on Jintan ceased relatively early with continued carbonate aggradation in adjacent smaller terminations (M1, M1-East). Spectacular reservoir architecture and diagenesis are revealed by the seismic. Several transgressive, aggradational, and progradational cycles are overprinted by repeated karst events. Dissolution features and bank-margin collapse are aligned to a deep-seated regional fault system, which periodically became reactivated during carbonate growth. A large triangular-shaped graben formed during one of the faulting periods but subsequently healed by a prograding reef-margin sequences. Two alternative scenarios are presented to explain the ultimate demise of the platform. The first proposes drowning resulting from a combination of subsidence and eustatic sea level rise. The second evokes a much-later drowning, which was preceded by a long period of exposure resulting from a second-order sea level fall and an initial decrease in subsidence caused by the onset of tectonism in Borneo during the late Miocene. In any case, following a hiatus of about 5 m.y., the platform was finally buried by deep-marine siliciclastics that prograded into the basin from the large delta systems of northwest Borneo. Recognition of growth architecture, faulting, and karstification is a key to exploiting the hydrocarbon reservoirs of the Mega Platform. A 30-m-thick low-porosity and -permeability layer shields the gas trapped in Jintan from the underlying aquifer. Penetrated by only one well, the extent of the layer and areas of breaching caused by faulting and karstification are identified on seismic. Interpretation of the seismic is critical to assessing whether and how the underlying aquifer is felt during reservoir depletion and whether there is pressure communication between adjacent reservoirs connected via the aquifer. Cores and logs from three wells provide ground truthing of reservoir architecture, karst features, and faulting derived from the interpretation of reflection and inversion seismic. The interpretation is then imported into static and dynamic 3-D models to constrain reservoir properties, predict dynamic behavior, and guide optimum field development.

Lower Miocene Nukhul Formation, Gebel el Zeit, Egypt: Model for structural control on early synrift strata and reservoirs, Gulf of Suez

The Aquitanian-early Burdigalian (lower Miocene) Nukhul Formation at Gebel el Zeit, Egypt, was deposited during early stages of Gulf of Suez rifting. The unit dips 8-15° less than underlying prerift strata, indicating that significant rotation and extension preceded subsidence of the Gebel el Zeit fault block. The Nukhul Formation at Gebel el Zeit is up to 75 m thick in outcrop and consists of a lower sandstone and an upper carbonate unit. The formation varies considerably along strike because of syndepositional differential movement of small fault-bounded blocks. The lower clastic unit at South Gebel el Zeit contains poorly sorted, conglomeratic, marly sandstone that commonly displays grading and Bouma sequences. Beds were deposited below storm base by sediment gravity flows. Thicker intervals are inferred to fill small, structurally controlled, submarine gullies that funneled sand and gravel southwestward to a half-graben basin. In contrast, an inferred correlative, thin, basal conglomeratic unit in North Gebel el Zeit was deposited in a shallow-marine setting. The presence of basement clasts in Nukhul strata indicates early syndepositional uplift due to structural tilting. The upper carbonate unit consists of bioclast, peloid, and intraclast packstone, wackestone, and grainstone with minor floatstone, rudstone, and coral-algal boundstone. Carbonate strata were deposited variously in deep-marine, low-energy peritidal and subtidal, and reefal environments. Deeper submerged blocks were the site of carbonate resedimentation or deeper shelf deposition. Reefs and shallow-marine bioclast shoals formed on higher submerged blocks. Nukhul strata show that synrift reservoir prediction in the Gulf of Suez, the Red Sea, and presumably in other rifts requires mapping of synrift cross faults and fault block by fault block facies analysis. Robert Winn is a consulting geologist. He received a Ph.D. from the University of Wisconsin-Madison in 1975 and joined Marathon Oil Company as a research geologist in 1977. He left Marathon in 1994 for the University of Papua New Guinea, where he was senior lecturer and then associate professor. He was head of the Geology Department from 1996 to 2000. His primary interests are in clastic sedimentology and sequence stratigraphy.Paul Crevello is a consulting geologist and technical director of PetrexAsia, which he formed in 1997. He received an M.S. degree from the University of Miami (1978) and a Ph.D. from Colorado School of Mines (1989). He was employed by Marathon Oil as a research geologist from 1978 to 1994 and by the University of Brunei as senior lecturer from 1994 to 1997. His specialties are in sequence stratigraphy and sedimentology of carbonate and turbidite systems and the integrated calibration of 3-D reservoir models. William Bosworth is employed by Marathon Oil Company. He joined Marathon in 1984 and has worked principally on international exploration and development projects. From 1980 to 1984 he taught structural geology and tectonics at Colgate University in Hamilton, New York. His research interests are principally in extensional tectonics, continental stress field evolution, and the paleogeodynamics of the African plate.

Porosity Evolution and Burial Diagenesis in a Jurassic Reef-Debris Reservoir, Smackover Formation, Hico Knowles Field, Louisiana

Hico Knowles Field, located in Lincoln Parish, northern Louisiana, provides the only example to date of Smackover production from a skeletal carbonate buildup (Fig. 24-1; Baria et al., 1982). Reservoirs in the Upper Smackover occur mainly in oolite grainstone shoal facies. Most carbonate buildups drilled to date in the Smackover trend have been either of nonreservoir quality or in water-wet legs of productive intervals (Baria et al., 1982).