Research On Reservoir Characteristics Of The Second Member Of The Upper Triassic Xujiahe Formation In Xiaoquan-Xinchang Area,Western Sichuang Depression

Xujiahe sandstone in west Sichuan of China which is not only the important gas reservoir and producing pay, but also the one of the most deeply buried and tightest clastic reservoir rock. The area of Xiaoquan-Xinchang is located in the tectonic belt of Xiao-Feng in the middle of western Sichuan depression. The second member of Xujiahe Formation is the key position for oil and gas exploration in this area, but he reservoir has experienced a complex change in the process of diagenesis, deeply buried and with the complex geological features, which has caused considerable deterioration and heterogeneity in the Xujiahe sandstones, and the main controlling factors of the development of the reservoir is not clear.This a series of factors restrict the oil and gas exploitation. In this article, using the theories of sedimentary, geology of petroleum and reservoir geology, sedimentation,reservoir characteristics and the development of reservoir affecting factors are comprehensively studied through core data, microscopy observation of cast thin members, analysis of physical properties, scanning electron microscope and X ray diffraction.We get some results and conclusions as follows:1Sedimentary faeies analysis indicats that the major phase is braid river delta. The majority of the area is delta plain and delta front, predelta is rarely seen. In the microfacies of distributary channel, the thick sandbody is overlay, which is mainly distributed in the2and5sand group of the second member of Xujiahe Formation. Underwater disrtibutary channel and mouth bar is developed in the2,4and7sand group of Xujiahe Formation.2Research on rock characteristis indicates the main lithology is feldspathic litharenite, lith quartz sandstone and lithic sandstone.The sandstone’s granularity in the area is medium, which is with fine separation, sub-angular, high textural maturity. The main types of cementation are compaction and pore. In the interstitial material, the matrix is little and the mainly cement is carbonate mineral, then is the silicon.3Research on diagenesis demonstrates that the main diagenesis of Xu2are mechanical compaction, pressolution,cementation,dissolution and cataclasis. In addition, according to mudstone vitrinite reflectance, the highest peak temperature pyrolysis, inclusion thermometry,clay mineral X-ray diffraction analysis, the characteristic of authigenic mineral assemblages and various kinds of diagenetic phenomenon analysis, the sandstone in the study area is in the late diagenetic stage, but some area is in the end of middle diagenetic phase B. In the end, based on the diagenetic mineral forming period analysis, we set up two kinds of diagenetic sequence. One is mainly chlorite pore lining diagenetic sequence, another is with the intense quartz secondary overgrowth diagenetic sequence, but without chlorite lining.4According to the analysis of thin member and scanning electron microscope (SEM), the study shows that there should be residual primary intergranular pore, solution enlarged pore, intergranula dissolution pore, moldic pore, intercrystal pore and fracture in the second member of Xujiahe Formation. The pore combination is divided into5types,namely as,solution enlarged pore combining with residual primary intergranular pore, dissolution pore combining with moldic pore, surplus intergranular pore with fracture,intergranula dissolution with intercrystal pore and mixing pore. The throat in the area is not well developed, and its main shape is lamellar, curve, necking and bundled. The holes are isolated with poor connectivity. Throats radius are small, and most of them are microscopic and fine.5According to the form of capillary pressure curves and the main parameter of pore structural distribution parameters, the study area is divided into four types of pore structure, I class for low threshold pressure and medium-fine throat, II type class for lower threshold pressure fine throat, III has middle threshold pressure with micro-fine throat, the type of Ⅳ has high threshold and its throat is microscopic. The pore texture in the area is complex and the parameters have a large range. In short, the reservoir has comparatively small median radius, high expulsion pressure and saturation degree of mercury, but with low efficiency of mercury exiting rock. The effective porosity proportion in the rock is little and seepage ability is poor.6The analysis of physical property of core shows that the porosity in the area ranges from0.34to12.12with an average of3.81, the permeability from0.001to0.968with an average of0.036millidarcy which belongs to tight reservoir. Among them,4,5and7sand formation of Xujiahe is best.7Through the above analysis,combined with the practical situation of the exploration and development of the area,the sandstone in2member of Xujiahe Formation is classified in to I,11,111and IV, which is evaluated best, better, bad, and IV is devided in to nonreservoir. The factors on the effect of reservoir are sedimentation, diagenesis, tectonism and so on.8At last, the profitable exploration target fields of the important member of4,5,7in Xujiahe Formation have been forecasted.The results show that the profitable exploration area in4sand group mainly distributes in the region of the well of Xin203-Xin201-Xin101-Xin chang8in the central area, and the area of Xin5-Xin6or around the well of Xin7. The profitable area in5sand group mainly distributes in the northern of well Xinchang7and Chuangxiao560, the southern area of well Xinchang8-Xin3-Xin301, the western region of the welll of Chuanke1-Xin10-Chuanxiao568or around the area of well Xinchang13and6which has many cracks. For the7sand group, the favorable zone located in the region of well Xin856-Xin853-Xin101-Xin2and in the southern of well Lian150.