Sedimentology And Lithologic Reservoir Exploration In Salt Lake Basin, Eogene Of The Southwest In Qaidam Basin

The study area, Southwest of Qaidam basin, in south part of the west Qaidam fault depression, is one of fuel-rich depressions of Qaidam basin. It includes the North Kunlun fault-step belt, Gasi fault-depression belt and Shizigou-youshashan fault-step belt etc. The Eogene in southwest of Qaidam basin, mainly consisted of clastic rock and mingled with thin beds of carbonatite, gyprock and saltrock, is a typical depositional framework of salt lake basin. The lacustrine mudstone (E₃²) of the upper segment within the lower Ganchaigou formation is the hydrocarbon source rock. This formation forms several reservoir-seal assemblages of self generation - self accumulation and lower generation - upper accumulation.Take Eogene of the southwest of Qaidam basin as the main target stratum, the formation condition, environment of palaeogeography and sedimentation evolution of salt lake basin were systemic studied and established the framework of sequence stratigraphy. The study shows that the medium cycle controls the formation of lithologic traps. After the study of lithologic reservoir types, key controlling factors and favorable accumulation condition in salt lake basin, the favorable reservoir zones, the prospecting exploration domains and zones are totally predicted. The above studies systemically guided the lithologic reservoirs exploration in salt lake basin.The salt sequence stratigraphy of Eogene in southwest of Qaidam basin are divided into 3 super-long-period datum level cycles or 9 long-period datum level cycles, the LSC1, LSC2 and LSC3 generated in the early developmental phase, LSC4 and LSC5, in the expand phase and LSC8, LSC9 in the extinct phase of lake basin. The Sequence assemblages controlled the distribution of lithologic traps. The favorable reservoir mainly situated in the following medium cycles, which are the asymmetry cyclic arrangement deepen upward in facies and inadequacy symmetry cyclic arrangement where the thickness of the upper hemicycle is much greater than the down hemicycle.The Altun Mt. front abrupt slope type depositional system (in Ganchaigou) and fault abrupt slope type depositional system (in Qigequan), in the border of Eogene salt lake of southwest Qaidam basin, are alluvial fan - fan delta — shore to meare short source depositional system. But in the Kunlun Mt. front, the gentle slope depositional systems (in Alaer-yuejin) are fluvial - delta -shore to meare long source depositional system. In salt lake basin, the shore to meare are the mingled deposition of fine grained clastic and carbonate, but in the depocenter (in Shizigou and adjacent area), the assemblage are gyprock, carbonate and mudstone etc. so various types of reservoir are formed in salt lake basin, and the porosity and permeability are different.The formation of lithologic reservoirs in Eogene of salt lake basin of southwest Qaidam basin are controlled by the distribution of source rock, the distribution of depositional system, structural background and migration pathway. The reservoirs within and under the source are effective for petroleum accumulation. The southeast slope of Gasi and slope of Yuejin 4 are the prospecting domains and the slope background and broad belt are prolific zones for lithologic reservoir exploration. Fine micorfacies analysis and reservoir lateral prediction show that clastic and algal limestone are favorable reservoirs for lithologic reservoir exploration.The innovative point of this dissertation is to study the sedimentation of Eogene in salt lake of the southwest in Qaidam basin with synthetic data of geology and geophysics for the first time and point out that the strand-bar sand bodies of clastic and algal limestone of carbonatite in salt lake basin are the main domains for lithologic reservoir exploration.