Relation of Hydrocarbon Reservoir Potential to Lake-Basin Type<subtitle>An Integrated Approach to Unraveling Complex Genetic Relations Among Fluvial, Lake-Plain, Lake Margin, and Lake Center Strata</subtitle>

Abstract

A relatively small range of lacustrine-facies associations record the complexly contingent interactions of a wide range of physical, chemical, and biological processes (climate, tectonics, sediment supply, vegetation, landscape evolution). Each lacustrine-facies association contains fluvial, lake-plain, lake margin, and lake center strata with characteristic hydrocarbon reservoir potential. The accumulation of these lacustrine-facies associations and their potential hydrocarbon reservoirs arise from interactions of typical ranges of rates of potential accommodation and sediment plus water supply and can be interpreted genetically as overfilled, balanced-filled, and underfilled lake-basin types. Fluvial-lacustrine lacustrine-facies associations (interpreted as forming in overfilled lake basins) generally contain reservoirs that are best developed in aggradationally stacked highstand clastic shoreline strata and occasionally in skeletal carbonate or charophytic algal lithosomes or in lowstand incised valley fills and lake floor fans (basinally restricted turbidite and mass flow deposits). These reservoirs tend to have low vertical permeability (Kv) because flooding surfaces are generally marked by decreased input of coarse sediment and increased subsidence. They have the lowest average net reservoir:gross interval of the three lacustrine-facies associations. They do, however, have the highest average porosity and permeability and contain the largest overall reserves, mainly in lake-plain fluvial strata. Fluctuating profundal lacustrine-facies association (balanced-filled lake basins) have reservoir facies that include lake floor fans, incised valley fills, and shoreline clastics or carbonates deposited during transgressions and highstands. These reservoirs tend to have the smallest lateral extent and lowest average recovery factor of the three lacustrine-facies associations; (based on reservoir and fluid properties), but do have good vertical and horizontal permeability (Kh) in highstand and transgressive systems tracts and the best Kv of all the lake-basin types. Evaporative lacustrine-facies associations (underfilled lake basins) contain reservoir facies that are best developed in transgressive sheetflood clastics, early highstand fluvial channels, and late highstand shoreline carbonate grainstones. Early carbonate and evaporite cements are common in these reservoirs, and there tends to be a wide lateral displacement of highstand from lowstand systems tracts. They do, however, have the best Kh (because of common erosion that enhances lateral connectivity) as well as the thickest net pay of all the lake-basin-type reservoirs (as they tend to occur at relatively large potential accommodation rates). Associated fluvial styles among the lacustrine-facies associations (lake-basin types) appear to vary systematically, as a function of sediment plus water supply relative to potential accommodation rates: perennial, high sinuosity streams are most common in overfilled lake basins, intermittent to perennial low-sinuosity streams in balanced fill, and a wide range from ephemeral sheetflood or multithread braided streams to perennial high-sinuosity streams in underfilled lake basins. Observations indicate that these associations of hydrocarbon reservoir and seal play elements occur in a wide variety of tectonic settings and ages, from continental rift to convergent foreland basins of the Cambrian to Holocene. Continued success in economic discovery and efficient recovery depend upon continued testing and elaboration of these concepts and a deeper understanding of the essential processes controlling deposition of lacustrine strata.