Petroleum Geology Characteristic And Prospect Of Maichen Sag In Central And Southern Beibuwan Basin

Maichen Sag is a new area of hydrocarbon exploration of Beibuwan Basin, which hasn’t obtained commercial breakthrough, being a low exploration degree. It’s located in south-central Beibuwan Basin and the northern margin of Hainan Island, which area is 2800km2, has similar distribution area to the high level exploration area, Wixinan Sag and Wushi Sag. In recent years, Jiangsu oilfield branch of Sinopec has deployed and drilled a number of exploratory wells in Xuewen block onshore of the Western Maichen Sag, including Xuwen X1,Xuwen X2,Xuwen X2 a,Xuwen X3 and Xuwen X6. Xuwen X1 well has got oil indicating from tight sandstone in the third member of Eocene Liushagang formation for the first time, which oil production is 5.7m3/d. A similar situation also prevailed in other exploratory, not only Xuwen X3 well has obtained low oil production indicating in the third member of Eocene Weizhou formation, but also Xuwen X6 well has got oil production which are 8.3 m3/d and 5.2 m3/d in the third member of Eocene Weizhou and Liushagang formation, respectively. The Exploration results fully indicate that Maichen Sag has basic petroleum geological condition for the migration and accumulation of hydrocarbon, which deserve further research on petroleum geology study and evaluation of hydrocarbon accumulating zones, thus providing a new basis for the next exploration deployment.According to regional geology, sedimentary structural evolution features, sequence stratum division, petroleum geology study, previous research and the result of hydrocarbon-generating simulation experiments, the organic geochemical characteristics of hydrocarbon source rocks, the petrologic characteristics of reservoir and the type of source-reservoir-cap assemblages were analyzed, particularly biostratigraphic analysis and stratigraphic classification and correlation in Yang 1 well. Combining the drilling data in adjacent area and Seismic geological information, taking regional petroleum geological features and crucial geological factor as the starting and breakthrough points, the paper further indicate and analyze the condition of Meichen Sag’s petroleum geology and the main controlling factors of Hydrocarbon accumulation. Moreover, the paper is based on Hydrocarbon accumulation key geological factors, such as source rocks, reservoirs, reservoir-seal assemblage, traps and carrier systems, which makes an intensive study on Hydrocarbon migration accumulation modes and hydrocarbon accumulating zones.The results have shown that fault activities is the key dominant factors of the hydrocarbon accumulation in Meichan Sag. The distribution of source rocks, reservoir petrophysics, the traps development, sealing condition and the passages for hydrocarbon migration and accumulation in Maichen Sag are directly restricted by the different-sized fault activities at different time. Moreover, the developing time, intensity, and cutting formations of faults decide the sedimentary, filling and its distribution characteristics of the main hydrocarbon generation depressions. The faults developed in different period cross and cut through each other, which resulting in the formation of fault-anticline traps,fault-nose traps, and fault block traps. The distribution characteristics of faults that contact the deep source rocks in the depression, and the time and space coupling relationship between the active time of faults and the formation time of traps control and influence the migration and accumulation of hydrocarbons.In short, based on the petroleum exploration and researching extent, this study summarized and established the possible models of hydrocarbon migration and accumulation,including the “lower-generation and upper storage” mode developed in southern fault zones, the “lower-generation and upper storage” and the “self-generation and self-bearing” modes developed in the central depression belt, the “lower-generation and upper storage” mode developed in northern slope belt. According to “source-control theory” and combining with the achievements especially the analysis of rich hydrocarbon generation sub-sags, the favorable petroleum exploration plays are lithological traps controlled by fans developed in Oligocene Weizhou formation of the downdropped block of southern fault belts, and structure traps and fault-lithological compound traps controlled by fault –nose, fault block and faults. The braided river delta updip pinch-out sandstone lithological traps developed in the third member of Oligocene Weizhou formation, the Weizhou formation stratigraphical overlap traps caused by overlapping deposited layer by layer in anticlines and synclines, and Eocene Liushagang formation structure traps controlled by antithetic faults in northern slope belts. Acquiring big commercial breakthroughs can be expected if we put the implementation of drilling in the evaluated and selected optimum exploration targets developed in those advantageous prospecting zones into practice.