Oil and gas accumulations in the Ordovician carbonates in the Tazhong Uplift of Tarim Basin, west China

Oil and gas fields of the Paleozoic cratonic basin in Tarim are mainly distributed in the Tabei and the Tazhong Uplifts. In the Tazhong Uplift, there are three sets of oil and gas stratum systems, of which the Ordovician traps are the largest in size and close to oil source beds with the most favorable conditions to form large-scale oil and gas fields. The Ordovician sediments in the Tazhong Uplift have experienced a stable platform and tableland margin development period. The Ordovician is divided into mudstone interval, interbedded mudstone and limestone interval, limestone interval, limestone and dolomite transitional interval and dolomite interval. Of these intervals, both interbedded mudstone and limestone interval and limestone interval contain source rocks, while limestone interval and dolomite interval are reservoir beds. Fault activities and denudation have an obvious impact on carbonate rock reservoirs. The major Ordovician trap types of the Tazhong Uplift include compressional fault block, fault anticline and buried hill, and regionally. The Tazhong Uplift can be generally divided into central fault horst trap belt, northern slope trap belt and southern slope trap belt. In view of the in-situ oil source in the Tazhong, oil and gas migration in the Tazhong Uplift is characterized by a short distance migration through sand bodies and unconformity surfaces laterally and along faults vertically. Based on this study, it is concluded that the favorable oil and gas accumulation zone should be the northern slope and the northwestern pitchout end of the uplift because these areas are located in the hydrocarbon migration pathway with weak tectonic activities and favorable preservation conditions. (C) 2003 Published by Elsevier B.V.