Comparison Of Charaeteristies Of Hydrocarbon Accumulation Between Bashituo And Manan Structural Belts In Markit Slope, Tarim Basin

The Markit Slop is an important area for petroleum exploration in Tarim Basin. Recently, the petroleum exploration of Bashituo structural belt has achieved great breakthroughs and the reservoir of well YB1in Manan structure belt has gained a certain degree of discovery. But the success rate of the overall exploration of Manan structure belt was relatively low. The critical factor was the hydrocarbon accumulation regularity in Manan structure belt. So the comparative study of hydrocarbon accumulation characteristics and the research work for the evaluation of favorable exploration zone in Manan and Bashituo structure belt had been launched, striving to achieve breakthroughs in exploration. By using the data of geology, drilling, geophysical and geochemical and so on, this article, guided by the theory of Petroleum Geology and Reservoir Geochemistry, applied the ancient fluid as the breakthrough point and developed a research work on formation development, crude oil and source rocks geochemistry, fracture development and evolution, hydrocarbon accumulation stage and period and the favorable exploration target evaluation. The mainly achievements are as follows:(1) By studying on interpretation of the seismic profile and structural evolution, the differences of structure characteristic between the Bashituo and Manan structure belts were:①the fracture structural style of Bashituo is mostly caprock slippage, while the fracture structural style Manan is a combination of middle-shallow cap rock slippage and vertical superposition of the stretching normal fault of the deep basement.②Manan structure belt developed Hetian ancient uplift in the late Caledonian-early Hercynian with the eroding of the Ordovician and Silurian-Devonian; However, the Bashituo structure belt, which was mostly in a subsident stage, accepted deposition and formed a situation of Manan structure belt being higher than the bottom of Bashituo structure belt.③the Caledonian-early Hercynian structural deformation concentrated in Manan structure belt and the late Hercynian-Himalayan structural deformation concentrated on the Bashituo structural zone.④the structural inversion of Bashituo structural happened in late Hercynian period while the one’s of Manan’s happened in Himalayan period.(2) According to the fluorescence spectra of oil inclusions, the characteristics of Phytane series, Sterane series, Tricyclic Terpanes series, the value of carbon isotope δ13C of the oil and source rocks in these two structural belts, it is comprehensively consider that the crude hydrocarbon source rock of the reservoir of well YB1in Manan structural belt comes from two sets of hydrocarbon source rocks--Cambrian lagoon facies and slope facies and their organic matter type and abundance are well. The crude hydrocarbon source rock in Bashituo structural belt mainly comes from Cambrian source rock, whose organic type and abundance are better, and secondarily comes from permo-carboniferous source rock, whose organic type and abundance are worse.(3) By applying system analysis method of fluid inclusions, combined with the analyses of hydrocarbon generating history and burial history, the main differences of the hydrocarbon charging events between the Bashituo and Manan structural belt were:①there were two oil charging events in Bashituo structural belt:The first oil charging was the early low maturity oil charging which happened in286-253Ma in late Hercynian period and the petroleum entrapment pressure was under19.94-21.54Mpa; The second oil charging was the late high maturity oil charging which happened in18-0Ma in late Himalayan period and the petroleum entrapment pressure was under20.43-55.70Mpa. The first charging oil originated from rock slope facies in Cambrian, and the second charging oil originated from rock slope facies in Cambrian and paludal facies in carboniferous-Permian.②There were three oil charging events in Manan structural belt: The first charging oil was the early low maturity oil in358-315Ma in middle Hercynian period and the petroleum entrapment pressure was in11.23-12.88Mpa; the second charging oil was the high maturity oil charging in278-218Ma in late Hercynian period and the petroleum entrapment pressure was from13.63Mpa to14.78Mpa; the second charging oil was the high maturity oil in21-0Ma in late Himalayan period and the petroleum entrapment pressure was20.09-61.45Mpa. The first and two oil sources came from the lower Cambrian lagoon facies hydrocarbon source rocks, but the third one came from the Cambrian slope facies hydrocarbon source rocks.(4) By analyzing the two structural belt accumulation elements and accumulation characteristics, it was considered that:There existed three types of reservoir-seal assemblages in the BaShituop structural belt. The reservoir were respectively the Donghe sandstone member of Lower Carboniferous Bachu Group, bioclastic limestone member of Upper Carboniferous Bachu Group, limestone member of Carboniferous Xiaohaizi Group. The cap rocks were mudstone member of Carboniferous Bachu Group, gypsum dolomite member and gypsum member of middle-lower Kalashayi Group, mudstone member of Permian Kupukuzhiman Group, respectively. Nowadays, there are multiple sets of mature to high mature hydrocarbon source rock in the Cambrian and Permian-Carboniferous. The hydrocarbon traps, which were large in quantity and good in types, begun to take shape in Early Carboniferous and basically formed in Late Permian. These traps had a good fracture communication between the hydrocarbon source rock and the reservoir, and the petroleum trapped by them mainly migrated vertically or laterally. The type of reservoir in Manan structural belt was Ordovician weathering crust karst reservoir. The cap rocks were mudstone section of Carboniferous Bachu Group and dense carbonate rocks of Ordovician. There were a set of mature to high mature hydrocarbon source rock in Cambrian Nowadays. The trap types were a combination of anticline-crack-karst. The traps begun forming in early Devonian and finished in early Permian. The communication between the hydrocarbon source rock and reservoir was well for the good fractures and karst transport layer between them. Finally, the main controlling factors of hydrocarbon accumulation in these two structural belt are decided by an effective hydrocarbon source rocks, a well developed condition of reservoir and cap rocks, an evolution of palaeohigh-Ancient slope, the development of karst reservoir, well formed faults and their good communication..(5) On the basis of preferably predicting the exploration targets in these two structure belts, the Manan structure belt were divided into three different potential exploration areas using the method of analyzing specific issues:north-central Markit Ⅲ block, north Markit II block and northwest Markit I block, the northern part of the Markit I block YB3well. The Bashituo structure belt were divided into two favorable exploration zone of north-central Bashituo blocks and west Bashituo block. The hydrocarbon accumulation model of these two structure belts was established and then the corresponding exploration aims and proposes are put forward. It is pointed out that the favorable structural lithological traps with good storage conditions developed in Upper Paleozoic. While Manan structure belt developed traps with favorable structure karst composite which were good for petroleum accumulation in Lower Paleozoic.