Accumulation characteristics and exploration practices of lithologic reservoirs at eastern margin of the Pre-Caspian Basin

The Carboniferous carbonate rocks at eastern margin of the Pre-Caspian Basin originally depositing at the passive continental margin of Eastern European craton are closely related to accumulation of lithologic reservoirs. Three kinds of lithologic reservoirs, i.e. sedimentary-related, diagenetic-related and unconformable reservoirs, have been found through exploration in recent years. Based on regional structures, single-well sedimentary systems tracts and reservoir diagenesis, it is believed that lithologic reservoirs are controlled by four factors, i.e. tectonic movement, sedimentary environment, diagenesis and systems tract. Seismic attributes and post-stack inversion were used to predict the reservoirs. The predictions were validated by well drilling in the slope zone. The Pre-Caspian Basin has abundant petroleum resources, 85-90% of which concentrate in Paleozoic carbonate reservoirs, especially in organic reefs and anticlines, underlying very thick Lower Permian salt domes. At its eastern margin, where favorable geologic setting exists as a result of the uplifting of Uralian orogenic belt at the late stage of the Early Permian Epoch, lithologic reservoirs have been successfully discovered and explored in recent years, and most of them distribute in the west slope in North Tluwa Oilfield. Previous researches often focused on pre-salt seismic data processing and imaging as well as pre-salt structural mapping and trap delineation, which all aimed at structural traps in this basin, e.g. organic reefs, anticlines and fault blocks, but fewer efforts have been made on genesis and prediction techniques of lithologic traps. This paper deals with sedimentary environments, diagenesis and tectonic evolution, so as to find out those controlling factors of lithologic reservoirs