Quaternary Comprehensive Provenance Analysis Of Eastern Qaidam Basin And Lithologic Trap Predication Of Main Gas Field Area

Eastern Qaidam Basin is a wide-gentle and inner-continental stable lacustrine depression basin without faults, and the main gas fields, such as“Tainan”,“Sebei”,“Yanhu”, are recognized as small-scale and low-amplitude anticline gas reservoir. With less and less structure traps are discovered, the important point of next step exploration is how to find the lithologic trap. The key point of finding lithologic trap is to determine the sandbody source and locate the pinchout points. Aiming at such problems, some research work have been done and achievements obtained are as follows:(1) Comprehensive provenance analysis. Not only by summarizing and analyzing research results of predecessors on the provenance analysis of Eastern Qaidam Basin, but by referring to the characteristic of modern stream distribution, and utilizing all kinds of analysis approaches,such as heavy mineral, outcrop, ratio of sand and strata, seismic, the result is obtained and shows there exist two main provenance systems for Quaternary sediment: One is main provenance-south Kunlun mountain,and its main part controls all the deposition of south slope, while its southwest branch-“Nabei”provenance impacts local deposition of“Luoyanshan-Nabei”Area. The other is subsidiary provenance-northeast Aimunike mountain,whose influence area is limited into the“Yanhu-Yabaer”。Meantime, the genetic type and material source direction of sandbody for main gas field area of north slope is recognized as follows: Firstly, the sandbody is hydrogenous not aeolian sediment, whose sedimentary facies background is shore-shallow lacustrine beach bar. Secondly, the sand deposit is originated from the south slope deposit which have been reformed and transported by the coastal current and lacustrine wave, but not from the denudation area of north Qilianshan Mountains which should experience a long-distance transportation and be across a sag.(2) Lithologic trap predication. Firstly, according to the regional geological setting and current gas field exploration survey, K9-K13 is judged as the main interval which develops the Lithologic trap for the main gas field area of north slope. And then, under the control of sequence stratigraphy framework and reservoir sedimentary facies background of beach bar, the fine sandbody correlation of six frame sections have been done, and lithologic pinchout developed around maximum flooding surface(K9 and K11). Finally, the lithologic pinchout is projected to the structure map of K13 reflection zone which is the bottom boundary of Quaternary,and it is concluded that the lithologic pinchouts mainly develop at the nose uplifts and slope areas. Specifically, there is six favorable development areas of lithologic trap, which is“Taidong nose uplift”,“Sedong nose uplift”,“Tuoxi nose uplift”,“Tuonan slope”,“Senan slope”,“Yanxi slope”.