Ordovician Structural－ Depositional Model and Prediction forProfitable Crack Reservoir of Carbonate Rock in Tazhong Area, Tarim Basin

Key words: 
• Tazhong area /
• Ordovician /
• structural-depositional model /
• cracking reservoir of carbonate rock /
• three-dimensional finite-element simulation

Abstract: Ordovician carbonate rock formation is an important target for oil and gas exploration. In this paper, on the basis of regional structure backgrounds, Ordovician lithologic intervals are divided in Tazhong area and, structural-depositional model is built up. With the help of three dimension finite element simulation method, the favorable areas of carbonate fracture reservoirs are predicted. The purpose of this paper is to give a direction for carbonate oil and gas exploration in Tazhong area. Conclusions could be summed up as follows:(1) The lithologic characters of middle- upper Ordovician in the major part of Ordovician uplift in Tazhong are dramatically different from that of adjacent slope, especially from that of slope of south Manjiaer depression in the north of Tazhong number one fault. The lithology of Ordovician in the major part of Tazhong uplift is dominated by carbonate rocks, and the lithology of middle-upper Ordovician is mainly sandy mudstone in the slope of south Manjiaer in the north of Tazhong number one fault.(2) Sedimentation during Ordovician in Tazhong area is controlled by intracratonic depression and Cratonic margin aulacogen, which are with different properties of prototype basin. The uplift and sag in intracratonic depression have prominent control to the sedimentation of Ordovician. Middle-upper Ordovician deposition in the major part of Tazhong uplift is controlled by the large scale anticline in the intracratonic depression formedn early Caledonian (the end of early Ordovician). The restrained platform facies are developed in shallow water in such places where platform clinoform, platform margin, semi-restrained platform (include platform flat, lagoon) and Carbonate morphologic units are found. The northeast boundary of Tazhong area is sharp margin controlled by the early movement of normal fault of Tazhong number one fault. The sedimentation of south Manjiaer depression in north Tazhong is controlled by Kuluketage-Manjiaer aulacogen, the sediments are clastic rocks with slope and deep water basin facies. Middle-upper Ordovician deposition of Tanguzibasi area in south Tazhong is controlled by the syncline of intracratonic depression, the sediments consist of clastic and carbonate rocks with deeper open platform facies. Lower Ordovician in Tazhong area is platform facies controlled by the intracratonic depression, and the structure-sedimentation is no apparent difference between the major part of uplift and adjacent areas.(3) The first favorable fracture zones of Ordovician carbonate reservoirs being predicted in the major part of Tazhong uplift include the following areas:Tz161-Tz44 area, Tz24-Tz26-Tz27 area, Tz45 area, Tz3 area, Tz52 area, Tz403 area, Tz2 area, Tz22 area, the east part of Tz35 area, the west part of number one fault and so on. Correlation the results predicted with the observations of core fractures and well logging evaluating, the fitness is about over sixty-six percent. So it is proposed that three dimension finite element simulation method is very efficient in predicting carbonate fracture zone.