Comparison Of Hydrocarbon Accumulation Characteristics Between Fore-arc Basin And Back-arc Basin

Fore-arc basin and back-arc basin in Burma are Cenozoic basins on basement of Mesozoic strata. Petroleum exploration of Chindwin Depression in Fore-arc basin and Shwebo Depression in back-arc basin began in the mid-nineteenth century, but the exploration degree of the depressions is low due to the technical limitation, which restricte oil and gas exploration largely and urgently to deepen. Based on the idea of petroleum accumulation system, according to the analysis of hydrocarbon accumulation conditions in Chindwin Depression and Shwebo Depression, the main controlling factors and patterns of hydrocarbon accumulation were summarized, which provides a scientific basis for the optimization of exploration targets; By comparing reservoir characteristics of the world typical fore-arc basin and back-arc basin, the hydrocarbon accumulation patterns of fore-arc basin and back-arc basin were summarized, and the similarities and differences were revealed, which has improved the theory of hydrocarbon accumulation to some extent. The main results and conclusions are as follows:1. Based on the analysis of the basic conditions of hydrocarbon accumulation in Chindwin depression, the main controlling factors of hydrocarbon accumulation were summarized, the results shows that source rocks, reservoir and cover conditions of the depression are favourable, hydrocarbon migration pathways of Yenan, Indaw and patolon is open, but crest fault in folding develop and the existing reservoirs in shallow layer may be damaged because of the intense tectonic movements, hence the preservation of shallow layer reservoirs are poor. However, the fold amplitude in Eocene and the following formations is relatively small, and it’s conducive to hydrocarbon’s accumulation. The type of traps in Yeyein construct in east of Chindwin depression is unconformity, and oil and gas accumulate in this place migration from the center of Chindwin depression, and the formation of reservoirs was controlled by the injection strength of oil and gas between Miocene and late Pliocene. The formation of anticlinal trap in Inyashe structure is multiphase and traps in shallow layer may be damaged, but traps in deep layers with better conditions and the petroleum migration conditions is similar with Yeyein construct, and the formation of reservoirs was also controlled by the injection strength of oil and gas between Miocene and late Pliocene. Three types of petroleum accumulation model in Chindwin depression can be summarized:Petroleum migrate steppedly along faults and sandstone and accumulate in anticlinal and faulted anticlinal traps in western lifting tructural belt of Chindwin depression; Petroleum migrate vertically and accumulate in anticlinal and faulted anticlinal traps in middle fold tructural belt of Chindwin depression; Petroleum migrate steppedly along faults and sandstone and accumulate in unconformable traps in eastern lifting tructural belt of Chindwin depression.2. Main controlling factors of petroleum accumulation in Shwebo depression were summarized by analysis of petroleum accumulation conditions. the results shows that source rock in Eocene and the following layers in Shwebo depression has been mature and TOC is low, but with big thickness and hydrocarbon generation conditions are good. There are four sets of favorable reservoir and seal assemblages, and the ability of accumulation and sealing is good. The formation of trap is the key for petroleum accumulation in Shwebo depression, early traps might be reformed and damaged by later tectonic movements, the later traps may be deficient in charge of oil and gas because of the formation time. Traps formatted in Eocene and more early with good preservation and new trap close to source rook are favorable locations for petroleum accumulation. Two types of petroleum accumulation model in Shwebo depression can be summarized:Petroleum migrate steppedly along faults and sandstone and accumulate in paleogene traps in southwest slopes tructural belt in Shwebo depression;Petroleum migrate steppedly along faults and sandstone and accumulate in Neogene traps in central tructural belt and its circumference in Shwebo depression.3. Basic petroleum accumulation characteristics of fore-arc and back-arc basin has been summarized by contrast accumulation conditions of typical basins. the results shows that source rock in fore-arc basin is unfavorable because of the low TOC and the sapropelic-humus type of kerogen and the low thermal evolution. However, a large thickness of rock sediment in fore-arc basin can accelerate the thermal evolution of organic matter and make up for shortage of low TOC and bad type of kerogen, and conduciving to a large number of hydrocarbons generated. There are several sets of favorable reservoir and seal assemblages in fore-arc basin, but unstable tectonic activity easily lead to poor continuity of reservoir and seal in the strongly deformed area. There are many type of trap in fore-arc basin, including unconformity traps, anticlinal and faulted anticlinal traps developed along the basin axial and structure-lithology traps. Preservation condition of reservoirs in fore-arc basin is poor, reverse fault and crest faults in fold may easily lead effusion of the oil and gas. Source rock in back-arc basin is better than fore-arc basin with higher TOC and type of kerogen, high heat flow in back-arc basin is conducible for the thermal evolution of source rocks. There are several sets of reservoir and cap combination in back-arc basin with good reservoir and capping capability. Fault block trap controlled by down fault is common in back-arc basin and anticlinal trap, faulted anticlinal trap and fault block trap controlled by reverse fault also developed in part of back-arc basin. Preservation condition of reservoirs in back-arc basin is better than fore-arc basin, however, oil and gas in reservoirs may effuse or redistribute in some back-arc basin because of tectonic compression or uplift of base in later stage.4. Three type of petroleum accumulation regular pattern can be concluded in forearc basin and back-arc basin by contrast of petroleum accumulation characteristics:i Quality of source rock in back-arc basin is better while fore-arc basion is poor,the ability of hydrocarbon generation is controlled by thickness of source rocks in fore-arc basin; ii Plate tectonic setting in fore-arc basin and back-arc basin is different,types of traps and migration pathways are controlled by tectonic setting in fore-arc basin and back-arc basin; iiiPreservation of reservoirs and late accumulation are controlled by late tectogenesis in fore-arc basin and back-arc basin.