Oil/Gas Reservoir Forming And Preserving Conditions Of Devonian-Carboniferous In Xiangzhong Basin

Hydrocarbon resources are very rich in marine carbonate rocks. In nature, about 50% hydrocarbon resources accumulated mainly in carbonate rocks. In the southern land of China, there are almost 200x104km2 carbonate rocks distribution area, with a great predominance of large carbonate rocks area, multiple layer systems, great thickness, widespread distribution and abroad exploration prospect, which is a very important base of energy strategic in China.According to the high evolution characteristics in Xiangzhong basin, the article was based on the hydrocarbon accumulation theory, in pride of multiple theory, such as the sequence stratigraphy, structural geology, carbonate sedimentology, lithology, reservoir geology and geochemistry, and thoroughly studied the reservoir-forming mechinsm and preservation conditions of Devonian-Carboniferous, preliminarily straightened evolution of tectonics and sediments, thermal evolution of source rocks, match relations among reservoir formation, destruction, second accumulation and late accumulation, emphasized reservoir preservation conditions, applied evaluation system of hydrocarbon-bearing preservation units to divide expect area in Xiangzhong basin, proposed favorable exploration target area which provides decisions referenec for subsequent exploration works.Especially in the Lianyan depression, reservoir forming conditions have been analysized intensively. The article investigated reservoir forming advantages of "low combination" system. It was considered that the main factors of controlling gas enrichment area are tectonic deformation intensity, scope and sedimentary system. Exploration results and analysis on typical gas reservoir shows that the area where the tectonic deformation is weak, and movement is slightly, gas preservation conditions are fairly good; and that reef-shoal sedimentary belts and ramp region between geosynclinal and platform areas are favorable areas of oli and gas reservoir forming and preservation.Based on general study, inclouding structural patterns and conditions of hydrocarbon generation, migration, accumulation,â… ,â…¡,â…¢prospects area are divided:I-the middle Lian Yuan depression,â…¡-the middle and east of ShaoYang depression, the east of Lian Yuan and Ling Ling depression,â…¢-other regions. In prospect I, the block of Yang Jia mountain and QiaoTou river are recently targets for breakthrough of petroleum exploration.Compared with predecessors' cognitions, the article has some characteristics and innovations in the following aspects:(1).Applying the principles and methods of Vail prime sequence stratigraphy, and in pride of profile exploration out-door and well drilling data,one sequence interface of I type and 9 sequence interfaces ofâ…¡type were marked out first time in Devonian. Referening to the sequence stratigraphy of predecessors' cognitions in Carbonate,Devonian-Carbonate in the Xiangzhong basin was divided into 3 sequences of II degree and 21 sequences ofâ…¢degree.(2). Development rules and interrelation of reefs and platform basin facies in Xiangzhong basin are studied thoroughly. And the viewpoints was proposed firstly that reefs of Devonian Qiziqiao and Shetianqiao formation are distributed in belts and platform basin may be inter-reef synclines formed by rapid growth of reef. In sequence, the formation, development and extinction of inter-reef synclines was closely correlate with formation and evolution of reef belts. This pattern has good comparability with modern Australian Great Barrier Reef. (3).The Xiangzhong basin developed in convergent plate boundary which is the binding site of two large collision blocks. And its structural patterns are very complex. As a whole, the key structural elements of the whole section are plough imbricate thrusts, closed linear folds and ejective folds. Connected with data of remote sensing, aeromagnetic and gravity, Lian Yuan depression was studied and divided into the west fold belt, the middle fold belt (the middle structural belt) and the east fold belt (the east structural belt). And the structural patterns and formating mechanism of different structural belts were discussed in the article.(4). The article proposed methods of trap efficiency study, and pointed out that match relations between source-reservoir-cap assemblage and structural patterns and the matching of trap formation and source rocks evolution are the firstly conditions of trap efficiency and dominate factors in determining exploration value. Marine layers in Xiangzhong basin experienced polyphase, multimode structural movements of late Indosinian. And their had features of inter-appearance of rise-and-depression, anticline-and-syncline. The stable negative elements and distributed area of hydrometric coal measure strata are favorable areas of oil and gas preservation. In the caprock distributed area of favorable areas of the stable negative elements, two kinds of traps are searched:traps of platform beach or platform terrace or complex lithology and structure,fault anticline structural traps.(5). Connected with analysis on homogeneous temperature of fluid inclusion and study of structural evolution, cognition on evolution of source rocks and hydrocarbon generating patterns has obtained the important breakthrough. The analysis on homogeneous temperature of fluid inclusion shows that, the source rocks in Xiangzhong basin had 3 major hydrocarbon generation and expulsion processes, and the "four-section hydrocarbon generation" model was set up, served as "triple-section hydrocarbon generation" stage and cracked gas generation stage.