Abstract Recent developments in sedimentology, diagenesis, and hydrocarbon exploration suggest that the recognition and interpretation of volcaniclastic sediments can significantly influence exploration methods, and the prediction of reservoir geometry and quality in volcaniclastic sequences. Volcaniclastic sediments are characterized by predictable changes in composition, texture, geometry and distribution, which can be used during both geologic and seismic interpretation. Interpretations based on volcaniclastic sediments help to better define the volcano-tectonic and paleogeographic setting that controls the deposition of associated siliciclastic and/or carbonate reservoirs. Volcaniclastics are also important to an understanding of the thermal history of a sedimentary basin and its deposits, and to evaluations of source-rock maturity and reservoir diagenesis. Seismic lithostratigraphic modeling and facies interpretations can be used to differentiate high-impedance volcaniclastic facies from associated siliciclastic deposits. In additon, laterally continuous "marker bed" pyroclastic-fall and large-volume ignimbrite deposits, characterized by continuous reflections, can be differentiated from discontinuous pyroclastic-flow deposits and lahars characterized by more discontinuous reflections. The characteristics of reservoirs in volcaniclastic sequences are controlled by the volcano-tectonic setting, eruptive mechanism, depositional system, composition, age, diagenesis, and thermal and burial history. Volcaniclastic lithofacies typically have predictable distribution patterns that control reservoir geometry. The reservoir quality of volcaniclastic sediments is controlled primarily by their diagenetic history, because volcaniclastics are composed largely of reactive and unstable minerals, with the potential for rapid and extensive changes during burial diagenesis. The abundance of unstable minerals commonly leads to the destruction of porosity by cemetation and compaction processes, but may also enhance porosity by grain dissolution. Successful efforts to find hydrocarbons in volcaniclastic deposits will depend on the coincidence of porosity preservation and generation processes, with the timing of hydrocarbon migration and entrapment.

ABSTRACT The Tulare reservoirs at South Belridge field clearly reflect the influences of both the regional tectonism of the active margin setting, and the numerous sedimentologic controls intrinsic to the depositional system. The large structure of the South Belridge anticline overprints hundreds of discontinuous reservoir sands to create a combination structural and stratigraphic trap. This is well documented by more than 6,100 active wells in the heavy-oil producing field. This giant oil field is projected to produce more than a billion barrels of oil, from shallow (400–1,400 ft) Plio-Pleistocene fluviodeltaic sands in a situation where the average daily production is less than 50 barrels per well. The overall depositional setting of the Tulare at South Belridge field is that of a prograding fluviodeltaic system. Depositional environments include lacustrine, deltaic, and meandering and braided fluvial systems, with six major lithofacies types recognized. The sand-body geometries show considerable variability throughout the Tulare as a function of the changing character of the depositional system. Reservoir quality (permeability and oil saturation) and producibility are directly related to depositional lithofacies. Reservoir flow-unit geometries and quality can be predicted from the depositional model.

ABSTRACT Late Cretaceous volcanics, in the form of small, isolated bodies of submarine extrusives, are common in Central and South Texas. Hydrocarbon traps associated with these volcanic mounds have been known for some time. This is a detailed case history of one locality (Zavala County) where the Upper Cretaceous Olmos Formation (Taylor Group) has been locally affected by over 30 individual volcanic bodies. The Olmos Formation crops out in Maverick County, Texas, and dips southeastward into the subsurface of Maverick, Zavala, Dimmit, Webb, Frio, LaSalle, and Atascosa counties. The texture, composition, and detailed stratigraphic relationships of the subsurface Olmos Formation of Zavala County were determined from the analysis of 342 electric logs and core samples from numerous wells in the county. The analysis established that the Olmos Formation was derived from highlands of the Laramide Uplift to the west and deposited by fluvio-deltaic systems in the easternmost part of the Rio Grande Embayment of the Gulf Coast Basin. The Olmos has been subdivided into six lithofacies (A to F, oldest to youngest) on the basis of the shape of electric log curves, grain size relationships, and petrographic data. The geometry of these lithofacies was clearly delineated by net-sandstone maps. Lithofacies A is a thin coarsening-upward sequence which is interpreted as a small fluvial-to wave-dominated deltaic deposit. Lithofacies B, a coarsening-upward sequence with minor mudstone breaks, was deposited as a nearshore bar sand. Lithofacies C is a coarsening-upward sand overlying an eastward-thickening mudstone and represents a large eastward prograding, fluvial-dominated delta. Lithofacies D is a coarsening-upward deltaic sequence similar to Lithofacies C but with a shift in the depositional axis. Fining-upward point-bar sandstones, mudstones, and coals of Lithofacies E are the laterally equivalent delta plain deposits of A, B, C, and D. Lithofacies F is a thin, transgressive sandstone formed by reworking as the underlying progradational system foundered and the shoreline moved westward. A detailed study of the net-sandstone isopachs of the Olmos Formation showed that there is a well-defined correlation between the isopach lows and the position of underlying Late Cretaceous volcanic mounds. The effect is best shown by Lithofacies C and E. The thinning of the Olmos Formation is thought to have been caused by the rapidly prograding fluvial-dominated Olmos depositional system which caused the thickest sands to be deposited away from volcanic topographic highs. Lower compaction rates over these volcanics undoubtedly contributed to the depositional thinning of the sands. The stratigraphic relationships produced by the thinning and draping of the Olmos Formation over the volcanics generated early potential traps for hydrocarbons. Continued differential compaction over the volcanics produced localized, generally northeast-southwest trending horsts and grabens over the volcanic highs, creating an additional trapping mechanism. Presently, there is a well-defined relationship between the location of producing wells and fields associated with the volcanic bodies and thinning and faulting of the Olmos Formation in Zavala County.