Regional Appraisal of Hydrocarbon Potential of Trans-Pecos Texas: Methodology and Conclusions: ABSTRACT; Expert Systems in Seismic Exploration: ABSTRACT; Geotectonic Evolution of Bering Sea Area, Alaska: ABSTRACT; Geology and Petroleum Potential of ... Association Round Table 249 eral is monocrystalline quartz and the matrix components are ferroan cal- cite, siderite, and kaolinite. Porosity appears primarily secondary in nature. Core analysis has shown 23% average porosity and 210 md per- meability. Oil-base cores indicate an irreducible water saturation of 44% and residual oil saturation of 22%. In the area of study, estimated reserves are 22.27 million m ' of oil in place. The produced oil has a density o f 8 4 4 k g / m , a viscosity of3,3 cp,anda gravity of 37° API. The produced oil is undersaturated in relation to gas and the initial reservoir pressure was 3,960 kPa. The absence of a gas cap and an active aquifer has resulted in implementation of a secondary- recovery waterflood mechanism based on a 5-spot injection pattern. Pri- mary recovery is estimated to be 10%, with an additional 31% from waterflood. DEJONG, H. W., SUNIT K. ADDY, GEORGE W. WHITNEY, and RALPH E. WORTHINGTON, ARCO Exploration Co., Denver, CO Regional Appraisal of Hydrocarbon Potential of Trans-Pecos Texas: Methodology and Conclusions Evaluation of large areas requires a different approach from that used to develop prospects. Regional cross-section networks generally ignore minor correlation problems. We examined and related all potentially sig- nificant parameters, and used reduced scales to subdue nonessentials and to cover a large area. Organization and planning must permit free inter- change of ideas and close cooperation among those working on the pro- ject. Regional structural and stratigraphic analysis of Trans-Pecos Texas strongly suggests that widespread trap-destruction by faulting and ero- sion led to hydrocarbon leakage and induction of fresh water into pro- spective zones, which reduced the likelihood of accumulation and preservation of economic reserves. At least 4 major periods of tectonism, pervasive fresh water, mineral- ization, basic igneous intrusives and extrusives, high heat-flow regime, increasing percentage of carbon dioxide southwestward from the Dela- ware basin, and contemporaneous vertical movement suggest that the opportunity for major reserves is minimal. A review of 1,000 mi of CDP seismic data from 4 areas disclosed 65 structural leads or traps, of which 45 had significant tests. All these tests were failures; many gave indications of sizable amounts of fresh water. None of the areas have been completely condemned. However, only relatively small reserves can be anticipated for nearshore to continental Jurassic and Cretaceous sections in the Chihuahua Trough, Pennsylva- nian and Permian reefs formed on a shelf environment or on the flanks of major uplifts, or in areas such as the Marfa basin. DENHAM, L. R., Seiscom Delta United Corp., Houston, TX Expert Systems in Seismic Exploration Artificial intelligence research has produced few practical results in most of its branches. However, "expert systems" in limited fields of expertise are potentially practical and cost-effective tools in many fields of exploration geophysics. Recent breakthroughs, such as writing expert systems in languages less exotic than Lisp, have made it possible to install a practical expert system on even the smallest computer. A recently pub- lished expert system written in Forth compiles a rule base into very com- pact code, and then uses it to reach decisions based on data supplied by the user. Such a system makes it possible for a small computer to be the geophysicist's advisor on many different subjects, because one expert sys- tem can use any number of rule bases. The expert system then becomes a practical tool for standardizing the decision-making process, even in comparatively trivial areas. DESAUTELS, DAVID A., Elf Aquitaine Petroleum, Houston, TX Geotectonic Evolution of Bering Sea Area, Alaska The geologic, structural, and tectonic history of the Bering Sea area since Paleozoic time is best viewed in terms of major plate-tectonic inter- actions. The geotectonic style of disparate areas is apparently related to the nature of plate motion at the time of tectonic imprint. Three major structural belts that have existed since the Mesozoic can be traced from the Siberian .sector across the Bering Sea and into Alaska. The northern belt, the Verkhoyansk-Chukotsk-Seward-Brooks, consists of miogeosyn- clinal sediments that were deposited beginning in earliest Mesozoic time. The middle belt, the Okhotsk-Chukotsk-Yukon-Koyukuk, consists of a Mesozoic magmatic arc and numerous allochthonous terranes, formed due to the convergence-subduction of a southern oceanic plate. The southern belt, the Koryak-Anadyr-Peninsular, consists of terranes accreted during Cretaceous time and forms the southern hmit of Meso- zoic subduction. During Late Cretaceous to early Tertiary time, these belts were oro- clinally bent southward by an east-west compressional event, causing the subduction zone to shift to a more southerly location, thus forming the current Aleutian Island arc system, behind which the fragments of 2 Cre- taceous oceanic plates were "trapped." These oceanic plate fragments may consist of an Early Cretaceous plate and a portion of the Kula plate(?), which carried a northward-migrating arc system. The hypothe- sized Early Cretaceous plate may have had a counterpart separated by a spreading ridge, both of which have been subducted beneath the Berin- gian margin. DESPRETZ, JEAN-MARIE, Petrofina, Brussels, Belgium, THOMAS E. DALY, REMI, Inc., Englewood, CO, and EDWARD ROBINSON*, Consultant, Kingwood, TX Geology and Petroleum Potential of Saba Bank Area, Northeastern Car- ibbean Recent exploratory activity on Saba Bank in the northeastern Carib- bean has provided geologic information showing that this frontier area possesses all of the attributes necessary for the commercial accumulation of hydrocarbons. The first well drilled in the area penetrated 9,370 ft (2,856 m) of sediments including 3,021 ft (921 m) of Eocene carbonates containing zones of good to excellent porosity. Geochemical studies show the presence of good but immature source rocks with the extractable hydrocarbons being migrated rather than indigenous, The geothermal gradient and vitrinite reflectance data indicated the threshold of the oil window would be reached around 10,000 ft (3,048 m). The second well was drilled to test a postulated reef on a basement high at a sufficient depth to fall within the oil window. The well bottomed in Eocene andesite at 13,881 ft (4,231 m). Reef carbonate was not encountered; the well pen- etrated turbidite sandstones and siltstones with low to moderate porosity and permeability. A test of gas shows recovered small amounts of C1-C5, but the formation is believed to have been badly damaged by severe mud loss during drilling. Geochemical studies confirm the presence of good source rocks. Reworked unmetamorphosed organic matter of probable early Eocene to Cretaceous age suggests that the Cretaceous cannot be considered economic basement in this area. Reinterpretation of the seis- mic data explains why the two wells were dry and indicates the presence of a submarine fan area, reefs within the oil window, and large structures in an area of thick sediments of probable Cretaceous age. DICKEY, RODGER L., Independent, Waco, TX Depositional and Diagenetic History of Buda Limestone in Central Texas and Its Relationship to Petroleum Potential The upper Comanchean Buda Limestone (Cretaceous) is a known res- ervoir for hydrocarbons in central Texas, producing from depths as shal- low as 700 ft. Understanding the character of the Buda Limestone and its complex depositional and diagenetic history is essential to developing a sound exploration strategy and to insure maximum production. In central Texas, the Buda Limestone may be divided into a lower, micritic facies, and a dense, in places dolomitized, intrasparite upper fades. The upper Buda is more porous and contains most of the produc- ible hydrocarbons in the formation. The upper contact is an undulating erosional surface, unconformably overlain by impermeable Woodbine shales. Porosity enhancement appears greater in areas of faulting and fractur- ing, especially where occurring along erosional drainage divides. Because of the apparent correlations between favorable structure and marketable oil production, economic prospecting methods should seek to delineate zones of faulting and fracturing along areas where the upper Buda was exposed to weathering.