The Study Of Sedimentology And Reservoir Characteristics In Chagan Depression

Chagan depression is an intra-continental pull-apart depression in the east of Yingen-Ejinaqi basin developed on later Paleozoic fold basement of Alashan continental block. It is a continental sedimentary faulted lake basin with multiple and near provenance fillings, also with various lithology and lithofacies as well. This paper uses theories of sequence stratigraphy, modern sedimentology,reservoir geology and laboratory methods to analyze drilling section, seismic data, petrological testing, core observation and hydrocarbon shows so as to study sedimentary facies, depositional system distribution and vertical evolution principle of the depression. There are six depositional systems classfied for the depression: alluvial fan, underwater fan, fan delta, fluvial, shore-shallow lake sand bar and deep-lake depositional system.The regional structural movements mainly controlled the depositional systems, their distribution and evolution. The depositional systems evolution can be approximately divided into three stages that reflect a full sedimentary cycle of a fault basin from formation development to shrinkage. The first member of the Bayingebi Formation deposited during the early period of the Early Cretaceous– an initial faulted depression stage, developed alluvial fan, fan delta depositional system. The second member of the Bayingebi Formation developed shallow and semi-deep lake depositional system during an intense fault depression stage; The Suhongtu Formation deposited in stable fault depression stage, developed shore-shallow lake and fan delta depositional system in the early - middle stage, alluvial fan and fluvial depositional system in the late stage; The Yingen Formation developed alluvial fan and fluvial depositional system, which was produced in the fault depression shrinkage stage. The characteristics of the Suhongtu Formation reflect shore - shallow and semi-deep lake sediments interbedded with volcanogenic rock.The sandstones as reservoir are mainly alluvial fan, fluvial, fan delta, underwater fan sediments, secondly shore-shallow lake sediments. This paper also uses lots of data of conventional rock thin sections, casting slices, scanning electron micrographs, mercury injection, for discussing reservoir feathers. Reservoir rocks are mainly feldspathic litharenite, lithic arkose and lithic sandstone with secondary and micro porosity and dissolved porosity of calcareous cement. The influence reservoir factors in this area are mainly terrigenous clastic components, sedimentary facies and diagensis.