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Enrichment Regularity of Hydrocarbon in Plateau Saline Lacustrine Basin

Abstract

Qaidam Basin has a more high altitude since Cenozoic because of the Tibetan Plateau continuous and periodical uplifting, which approximately has five intensive uplift periods including A(58Ma-53Ma), B(45Ma-30Ma), C(25Ma-20Ma), D(13 Ma-7Ma) and E(5Ma-Now). Uplifting activities cause a closed, dry and cold environment and abundant salt source for the plateau saline lacustrine basin. Carbonate, sulfate and chloride usually precipitate with dark mudstone and all geochemical indicators that salt rocks develop in relatively deep water. Although biological species would decrease with higher salinity, halophile would massively propagate. Saline lake ordinarily can stratify due to the different salinity to form a halocline, the enclosed water below which is anoxic, in favor of preserving organic matters. Organic matters cumulate with the salt in the zones, where usually have hydrodynamic breaks due to the double-diffusive convection. The Toc of source rocks is widely lower, nevertheless excellent kerogen types and high hydrocarbon conversion rate still can make them as high-quality source rocks. Plateau saline lake has a more favorable environment to develop sand bodies of underwater distributary channel, estuary bar and beach bar than in freshwater because of the relatively higher viscosity and salinity, which usually make river channel swing, detritus not deposit in the stream outlet, and suspension components be transported farther. Those favorable sand bodies usually distribute below the slope break blets, where the velocity of flow changes suddenly. It is easy to develop lithologic traps because of the reclusive fluid system and strong cementation. The analysis on homogenization temperatures of oil and gas inclusions indicates that there are at least two stages of obvious migrations of oil and gas, respectively corresponding to the intensive uplift period C and D. Meanwhile, boiling inclusions in the deep show that the migrations of hydrocarbon fluids relate with the uplifting movements. It means the main hydrocarbon accumulation dynamics is tectonic stress, so the key point of hydrocarbon accumulation is the carrying capacity of fracture-sand conducting system. It is obvious that slope break blets controlling the distribution of favorable sand bodies and hydrocarbon accumulating along the fractures in plateau saline lake basin. In sum, the favorable areas for lithologic reservoirs are below the slope break belts with rich conducting fractures.