H. Scott Hamlin

Depositional Controls on Reservoir Properties in a Braid-Delta Sandstone, Tirrawarra Oil Field, South Australia

The Tirrawarra Sandstone contains 146 million bbl of oil in Tirrawarra field in the Cooper basin of South Australia. We used core, well logs, and petrophysical data to construct a depositional-facies-based flow-unit model of the reservoir, which describes rock properties and hydrocarbon saturations in three dimensions. Using the model to calculate volumes and residency of original and remaining oil in place, we identified an additional 36 million bbl of oil in place and improved understanding of past production patterns. The Tirrawarra Sandstone reservoir was deposited in a Carboniferous-Permian proglacial intracratonic setting and is composed of lacustrine and fluvial facies assemblages. The stratigraphic framework of these nonmarine facies is defined by distinctive stacking patterns and erosional unconformities. Mudstone dominated zones that are analogous to marine maximum flooding surfaces bound the reservoir. At its base a progradational lacustrine-delta system, composed of lenticular mud-clast-rich sandstones enclosed in mudstone, is truncated by an unconformity. Sandstones in these lower deltaic facies lost most of their porosity by mechanical compaction of ductile grains. Above the unconformity a braid-delta system, dominated by aggradational bed-load channel-fill sandstones, forms the core of he reservoir. Sediment reworking by channel migration and locally by shore-zone processes created quartz-rich, multilateral sandstones, which retained the highest porosity and permeability of all the reservoir facies and contained most of the original oil in place. The braid delta is erosionally truncated and overlain by a more proximal braid-plain system. Braided-channel sandstones, however, are overlain by lenticular meandering-channel sandstones, which in turn grade upward into widespread mudstones and coals. Thus, this uppermost part of the reservoir displays a retrogradational stacking pattern and upward-decreasing reservoir quality. On the basis of the systematic arrangement of facies within this stratigraphic framework, as well as facies-related differences in composition and texture, we identified reservoir flow units that have characteristic petrophysical properties. Our results demonstrate that depositional variables are the primary controls on reservoir quality and productivity in the Tirrawarra Sandstone.

Ozona sandstone, Val Verde Basin, Texas: Synorogenic stratigraphy and depositional history in a Permian foredeep basin

The Ozona sandstone is a record of Permian synorogenic sedimentation in a foreland basin. The Ozona comprises terrigenous clastic slope and basin systems overlain by mixed clastic-carbonate shelf-ramp systems. Ozona sandstones form low-permeability gas reservoirs in Crockett County, Texas. I used wire-line logs and cores to map Ozona genetic stratigraphy and to reconstruct the depositional and tectonic history during the final phase of the Ouachita orogeny. Ozona depositional systems are composed of sandy turbidite channel and lobe genetic facies enclosed in laterally extensive muddy turbidite sheets and hemipelagic drapes. Channel and lobe complexes and turbidite sheets together form basin-floor apron systems. Coeval slope systems are mud-dominated products of mass transport processes. Sediment dispersal systems evolved from point sourced to line sourced.Ozona sequence development was primarily controlled by tectonic uplift and subsidence. Stratal geometries, facies associations, sediment input patterns, and depocenter locations are stable within sequences, but they change across sequence boundaries in response to tectonically driven changes in basin geomorphology. Onlapping stratal geometries in the lower sequence record excess accommodation space in the study area as plate convergence progressed from south to north. Offlapping strata in the middle and upper sequences formed in response to reduced accommodation space and intraforeland uplift in the north. At the base of each sequence, sandstone depocenters step toward the thrust belt in response to thrust-sheet loading and foredeep subsidence, but within each sequence, depocenters migrate away from the thrust belt. Foredeep migration through time provides a predictive tool for locating Ozona reservoir analogs farther south in Val Verde Basin.

Interaction of Burial History and Diagenesis of the Upper Cretaceous Frontier Formation, Moxa Arch, Green River Basin, Wyoming

Abstract Low-permeability sandstones of the Frontier Formation produce gas from reservoirs along the south-plunging Moxa Arch in the Green River Basin, southwestern Wyoming. Petrographic examination of 247 Frontier thin sections indicates that the main causes of low porosity and permeability in these sublitharenites and litharenites are compaction and cementation by quartz, calcite, and clay minerals. Major differences in burial history between the northern and southern ends of the arch have resulted in an average of 5% quartz cement in Frontier sandstones at the north end, compared with 13% at the south end. Burial histories of four wells along the north-south trend of the Moxa Arch and one well off the arch in the deep Green River Basin were reconstructed using stratigraphic data from well logs. Uplift and erosion events incorporated into the burial-history curves occurred during the Late Cretaceous, at the end of the Cretaceous, and from the Oligocene to the present. Modern geothermal gradients of 1.7 to 2.0°F/100ft (3.1 to 3.7°C/100 m) were used to model thermal maturity. Second Frontier sandstone on the La Barge Platform at the northern end of the arch (T28N, R113W) has a calculated Time-Temperature Index (TTI) value of 30, compared with a TTI of 334 in Henry field at the southern end of the arch (T13N, R113W). Second Frontier sandstone in the deep basin well (T22N, R106W) has a TTI value of 3079. In the five wells for which TTI was calculated, an excellent correlation (r = 0.95) exists between thermal maturity of the sandstone (log TTI) and average volume of quartz cement. Frontier sandstones at the southern end of the Moxa Arch and in the basin were buried to greater depths and exposed to higher temperatures than those at the northern end, so they experienced more stylolitization and intergranular pressure solution of chert, thereby liberating more silica for quartz cement. However, TTI is not a good predictor of reservoir quality because porosity and permeability in Frontier sandstones are controlled by many other variables in addition to quartz cement volume.

Fluvial Depositional Systems of Carrizo-Upper Wilcox in South Texas: ABSTRACT

In the Rio Grande embayment of south Texas, the Carrizo-upper Wilcox interval (Eocene) consists of two sand-rich coastal plain fluvial depositional systems that grade basinward into several deltaic complexes. The bedload channel system is dominated by multi-story, multi-lateral, fluvial, channel-fill sandstones. This system is typically > 90% sandstone. Shales are thin and laterally discontinuous, the remnants of abandoned channel fills. Bedload channel sandstones dominate the major fluvial axes and form the depositional framework of the interval. The mixed alluvial system consists of a more typical suite of coastal plain facies. Mixed-load channel-fill sandstones tend to be isolated and surrounded by overbank shales and thin sandstones. Crevasse splay and lacustrine f cies occur in the flood-plain area. Total-interval isopach patterns, sandstone geometries, and depositional systems distributions indicate that fluvial sediment input was converging upon the embayment from the west, northwest, and north. Economically, the Carrizo-upper Wilcox of south Texas has a threefold significance. The updip Carrizo sandstone is a major source of fresh groundwater, includes several large oil fields, and also contains deposits of uranium minerals. The downdip upper Wilcox trend is an area of active hydrocarbon exploration. End_of_Article - Last_Page 1465------------