Abstract Improvements in base-salt and subsalt seismic data quality have made it possible to recognize a spatial relationship between numerous salt structures, base-salt faults, and major subsalt rift basin reservoirs, and to develop geoseismic models for the prediction of subsalt fault-controlled rift basin reservoirs. The spatial relationship between salt structures and base-salt faults observed in many Europe and North American basins has led to the suggestion that faults and the resulting base-salt displacement, slope variations, thermal anomalies, paleotopography, and differential subsalt sediment compaction may contribute to a base-salt fault trigger mechanism which can control the initial location, trend, and morphology of salt structures. Consideration of the spatial salt structure-base-salt fault association in combination with a plausible base-salt fault trigger mechanism suggests that a genetic relationship may exist between many base-salt faults and salt structures which can be used to develop predictive models of subsalt structure and fault-controlled reservoir distribution. Models of initial salt structure development based on seismic and well data from Europe and North America summarize the effect of both presalt and post-salt normal, strike slip, and transverse faults that displace the base-salt, as well as faults that cut into the overburden. Similarities between the various models are integrated into a generalized model which can be used to predict the location and type of subsalt faults, and extend the interpretation of subsalt faults to poor seismic data areas and new areas. Since subsalt rift basin faults may have also controlled synrift sedimentary and diagenetic facies distribution, the salt structure-base-salt fault model can also be used to identify areas with the best subsalt synrift-postrift reservoir potential. Drilling results have confirmed subsalt fault and reservoir predictions based on the salt structure-base-salt fault model. Given the numerous rift systems overlain by evaporites with salt structures, the model should lead to the development of a relatively low risk-high potential subsalt reservoir prediction strategy that can be applied to other rift basins with good source potential that are sealed by salt.

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 Plio-Pleistocene nonmarine volcanic sandstones of the Cagayan basin, Philippines, have been significantly altered by early dissolution and cementation processes. The amount and type of alteration vary by formation, depth, and age of the deposit. Plio-Pleistocene fluvial sandstones (litharenites and feldspathic litharenites) buried to depths of 400–900 m, are only slightly compacted, but contain significant amounts of authigenic pore-lining clay and zeolites. Dissolution of plagioclase, heavy minerals, and volcanic rock fragments has occurred in nearly all samples, dissolving up to one-half the framework grains and increasing thin-section porosity to as much as 40%. The overlying Pleistocene sandstones are compositionally different (lithic arkoses and arkoses) and have not been as extensively affected by diagenetic processes. The more extensive alteration of the Plio-Pleistocene sandstones reflects increased diagenetic alteration with burial depth and time as a result of relatively high porefluid flow rates in shallow alluvial deposits. The diagenesis of the Cagayan basin Plio-Pleistocene sandstones indicates that significant secondary porosity can develop in nonmarine volcaniclastics as a result of early silicate dissolution during shallow burial diagenesis. Early dissolution and secondary porosity development have important implications for studies of nonmarine volcaniclastics. Early dissolution processes distort provenance, tectonic setting, and depositional environment interpretations based on the detrital mineralogy of older volcaniclastic sediments. Secondary porosity increases the reservoir quality of volcaniclastics prior to more extensive compaction and cementation. Recognition of similar shallow volcaniclastic reservoirs in the past may have been limited because of low resistivity sand identification problems caused by authigenic smectite.