Study Of Reservior Characteristics And Reservoir Prediction Of Changxing Formation In Jiannan Area

As favorable reservoirs, Carbonate reef and bank can form giant and supergiant field. In recent years, Along with the rising of the exploration degree of the Permian Changxing formation reef-bank reservoir in Sichuan basin, organic reef and bank has been found in successively in Jiantianba, Panlongdong, Huanglianxia, Yuanba and other regions, and Changxing reef has been paid more and more attention. There are more than30points of organic reef (including outcrops and downholes) in eastern Sichuan and western Hubei. The drilling data of Jiannan area reveals the existence of reef in northern Jiannan and bank in southern Jiannan. The distribution and new reef-bank reservoirs needs further research and exploration. Therefore, carrying out research on the distribution of Changxing formation of the Permian reef-bank reservoir in Jiannan area, precisely depicting the developing characteristics of reef-bank reservoir, as well as predicting and locating distribution area of new reef-bank reservoir has both important theoretical and practical significance.In this paper, following basic theories of carbonate sequence stratigraphy, sedimentology, reservoir geology, logging geology and seismic sedimentology, based on fine observation of outcrops and cores, combining with rock slice, logging, seismic and testing data, we carry on study on Changxing formation reef-bank reservoir by methods of combination of macro and micro facies and combination of geological theory and seismic technology. And several conclusions have been made:1.Based on the observation of outcrops and cores, combing with seismic data, two types of sequence boundaries were recognized, and Changing formation was divided into two third-order sequences. Sql was corresponding to first member, Sq2corresponding to the second member, of which transgressive systems tract (TST) and highstand systems tract (HST) were developed. Changxing formation was in unconformity with and the underlying Longtan formation and the overlying Feixianguan formation, of which thickness is180-340m. Reef and bank were mainly developed in HST.2.According to the sedimentary characteristics of the Changxing formation, there are two kinds of depositional models:"ramp model" and "platform model". The sedimentary pattern of Changxing formation is mainly the latter, which can be furtherly divided into4types of sedimentary facies:continental shelf, platform margin slope, platform margin reef and open platform, and interplatform bank, as well as several microfacies:platform bank, platform margin bank (reef), etc. The deposits are mainly bioclastic limestones (or dolostones), reef limestones, grain limestones (or dolostones) and the grain limestones (or dolostones). And the platform margin facies is the most favorable formation and evolution sedimentary facies belt of reservoir.3.According to much fine observation of core and rock slice, reef dolostones, grain dolostones, crystalline grain dolostones, and bioclast limestones were identified as main rock types of reservoir. Reservoir space was composed of intergranular dissolved pore, intragranular dissolved pore, intercrystalline pore, intercrystalline dissolved pore and fracture. Physical property data shows reservoirs in Changxing formation belongs to low-porosity and low-permeability reservoirs, physical property of dolostone is generally better than that of limestone. Through various mercury physical parameters analysis, reservoir throats in the study area are identified as micro and medium throat mainly; and reservoir properties has some correlation with the average pore-throat size, sorting coefficient and skewness.4.The development of reef and bank reservoir is strictly controlled by sedimentary microfacies, rock type, sea-level changes and diagenesis. Reef and bank is the most favourable sedimentary microfacies; hermatolith and grain dolostones are the main rock types; reef and bank are developed at the stage of sea level falling; the formation of reservoir is modified by diagenesis, in which destructive diagenesis include compaction, recrystallization and cementation, constructive diagenesis being dolomitization, dissolution and cataclasis. Almost all rocks of reservoir were dolomitized, physical property were improved by dissolution and cataclasis. Therefore, reservoir of Changxing formation with low porosity is at late diagenetic stage.5.Based on the geological study and seismic research, favorable reservoir development area were recognized by optimizing proper method of impedance inversion. It indicates that reservoirs were mainly distributed in the reef-bank of Jianzhuxi in the middle of study area, including well Jian431area and western Linxichang, while dolostones of the second member deposited. And when reef-bank developed, reservoirs were mainly distributed in the middle of Jianzhuxi, central-southern Jinzhuping, the high position of paleogeomorphology in north, Linxichang organic reef and partial high position of well Jian47area. Reservoir thickness is calculated by selecting threshold value of acoustic impedance inversion body. The result shows that middle Jianzhuxi, northern exposed beach and eastern Linxichang organic reef have bigger reservoir thickness. The distribution of reservoir thickness is highly in accordance with the distribution regularities of sedimentary facies.6.On the basis of these studies, we make predictions of favorable reservoir zone of Jiannan area. By making a comprehensive analysis of Changxing formation reef distribution, dolomite reservoir thickness map, dolomite reservoir thickness distribution forecast maps and other single factor, and taking into account of the development of reservoir properties and structure-sensitive position, the favorable zones for the Changxing formation reservoir is finally determined. We predicte class I, II and IE reservoir development zones, among which the class I is the most favorable reservoir development zone.