AAPG Geoscience Technology Workshop

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Characteristics and genesis of two categories of deep petroleum systems


Global and domestic deep petroleum exploration has made important progress in the past ten years. In the past ten years, we try to established refined geological modeling of Chinese typical marine and nonmarine basins, and spatial representation of deep petroleum systems by integrating geochemical and reservoir data to constrain generation, migration and accumulation process of deep hydrocarbon. Case studies include marine and nonmarine basins, the former are Baxian sag and Qikou sag in the Bohai Bay Basin, the Xujiaweizi sag in the Songliao Basin, the Mahu sag in the Junggar Basin, and the Kuqa foreland area in the Tarim Basin; the latter are the cratonic area of Tarim Basin, the eastern and central Sichuan Basin. Petroleum system analysis shows there are two categories of deep petroleum systems, which indicate significant differences between the marine and lacustrine deep petroleum systems. Category1: There are no significant oil cracking gas accumulation in the deep and ultra-deep zone, such as the Bohai Bay Basin and the Junggar Basin. The deep hydrocarbon are mainly humic gas and condensate generated from organoface D/E and F source rocks. The reasons without significant oil cracking gas are the relatively poor top seal, active faults, and high oil expelled efficiency of the source rock intervals in lacustrine basins. Category 2: Oil cracking gas are predominated in the deep and ultra-deep zone of marine basin, such as the Lower Paleozoic marine strata (Cambrian and Sinian) in the Sichuan Basin, the Lower Paleozoic in the Tarim Basin (Ordovician and Cambrian). Rich pyrobitumens are existed in the Cambrian and Sinian reservoirs in the Sichuan Basin, which indicate large-scale crude oil cracking into gas. High quality caprocks(marine carbonate rock , gypsum and salts) separate oil and gas systems of different eras in Sichuan Basin, which made deep and ultra-deep oil and gas can only vertically migrated into the middle and shallow layers in the very limited areas. The cracking degree of crude oil in the deep reservoirs in the Tarim Basin varies greatly, present gas and condensate in the Tazhong and Tabei uplifts are mainly migrated from deeper oil-cracking kitchen. We believe that not all basins can find significant oil cracking gas in deep layers, and high-quality top sealing conditions are the main reasons for the deep accumulation of oil cracking gas in sedimentary basins. In general, deep-formed natural gas migration forwards generally cannot reach medium-shallow reservoirs, which indicate that deep gas exhibits high-efficiency accumulation.