Analysis On Hydrocarbon Charging Process Of Silurian In Tarim Basin, Northwest China

Tarim Basin is a superimposed basin developed on the pre-Senian metamorphic basement. Silurian, the target strata of the thesis, is a suit of clastic strata, main composed of delta, littoral, tidal-flat and shelf facies. There were three typical palaeo-structural background in the late Ordovician, the uplift erosion zone, the slope transitional zone, and depression zone. The body strata main locates in the north depression and overlaps to the palaeo-uplift nearby. Now the rest area is about23.6×104km2. The Silurian strata has aroused the petroleum explorers’ interest by its widespread asphalt sandstones, many kinds of petroleum shows and industrial petroleum flow in some area.However, the Silurian strata is now covered by the upper Paleozoic-Mesozoic group strata and subjected to the superimposed and reconstruction of the multistage tectonic movements along with the basin since the late Paleozoic. The petroleum reservoirs experienced multistage charging, destruction, adjustment and preserved in the complex accumulation systems. It is the uniqueness and complexity that constrains the further understanding of the petroleum accumulation rules of the Silurian reservoirs.Based on the theory of the petroleum geology and the research on the diagenetic sequence, the fluid inclusion systematically analysis method and other geology, drilling, seismic and other geochemesry data, from the Silurian strata of the whole basin, in the background of tectonic evolution and sedimentary filling characteristics, focusing on the petroleum charging history, the thesis is main to discuss the complex petroleum accumulation process of the multistage charging, deconstruction, adjustment and recharging of different areas under the multistage tectonic movements. Combined with dissection of typical petroleum reservoirs of the North Tarim, Central Tarim and Southwest Tarim, the thesis will discuss the main factors, which control the petroleum accumulation, and then give the petroleum accumulation models of different areas, which can provide the related theory bases for the petroleum exploration of the Silurian. The thesis main gains the following conclusions:(1) Through the comparative analysis of the saturated hydrocarbon and carbon isotope characteristics of the oily extraction from the representative wells of different areas, the oil from the Silurian of North Tarim and the north slope of the Central Tarim is similar to the oil from the Ordovician of Tahe oil field in the both aspects. So their source rocks may have a good affinity. The oil of the Well Batan5in the southwest Tarim has the similar carbon isotope composition with the oil above, but it also shows some differences in the saturated hydrocarbon composition with the oil above. So it can be concluded that the oil of the Silurian strata of the Southwest Tarim may be from the source rocks of the reducing marine environment and the parent material type may be have some differences with the source rocks above. Based on the former researches, it is concluded that the two suits source rocks of the Cambiran-Ordovician both accounts for the oil of the Silurian of the Tarim basin.(2) Fluid inclusion results in different areas show that:the reservoirs in North Tarim have experienced four charging episodes and three charging phases. The first phase occurred in440.0-400.0Ma, represented by the widespread solid bitumen; the second phase occurred in247.8-220.2Ma, mainly represented by the oil inclusions of blue-green and yellow-green fluorescence; the third phase occurred in14.5-2.8Ma, mainly represented by the oil inclusions of blue-green fluorescence. The reservoirs in the Southwest Tarim have experienced three charging phases, the first phase occurred in the440.0-422.0Ma, represented by the widespread solid bitumen; the second phase occurred in261.9-254.6Ma, mainly represented by oil inclusions of blue-green fluorescence, the third phase occurred in8.2-1.1Ma, mainly represented by oil inclusions of blue-green fluorescence. The reservoirs of the north slope in the Central Tarim uplift have experienced five oil charging episodes and three oil charging pahses and one gas charging phase, the first oil charging phase occurred in440.0-395.OMa, represented by the widespread solid bitumen and the oil inclusions of blue-green fluorescence hosted in the early cracks (not through the grain) and secondary concrescence of quartz edge, the second oil charging phase occurred in265.2-213.6Ma, mainly represented by the high mature oil inclusions of blue-green and yellow-green fluorescence and supplemented by the low mature oil inclusions of orange-yellow and orange-red fluorescence, the third oil charging phase occurred in4.6-0.4Ma, mainly represented by the oil inclusions of blue-green fluorescence, the only one gas charging phase occurred in5.2-0.3Ma.Combined with the generation and expulsion history of the Cambrian-lower Ordovician and middle-upper Ordovician source rocks of different areas in the basin, it can be presumed that the first phase oil is mainly from the Cambrian-lower Ordovician source rocks; the second phase high mature oil of blue-green and yellow-green fluorescence is mainly from the Cambrian-lower Ordovician source rocks and the low mature oil of orange-yellow and orange-red fluorescence is mainly from the middle-upper Ordovician source rocks, this phase is the main accumulation stage; the third phase oil of blue-green fluorescence is mainly from the middle-upper Ordovician source rocks and the natural gas is the product of the Cambrian-lower Ordovician source rocks.(3) Synthesizing the results of fluid inclusion and dissection of typical petroleum reservoirs, it can be concluded that whether the petroleum will accumulate in the reservoirs of Silurian in the Tarim basin is constrained by three factors:①advantage palaeo-structure, the low potential region is apt to develop petroleum reservoirs. The asphalt sandstones mainly distribute in the North Tarim, Central Tarim and Southwest Tarim uplift. So the paleo-structure control the migration direction of the petroleum.②the fault connecting the reservoirs of Silurian and the source rocks or the petroleum reservoirs of underlying or the adjacent region, the petroleum of the Silurian mainly comes from the Ordovician-Cambrian source rocks. But between the Cambrian and Silurian strata, there are several suits of mudstone. So the fault is the main pathway for the vertical migration of the petroleum. The well and fluid inclusions results both show that the region near the fault usually has good petroleum showing.③the quality of the cap rock, cap rock is a main factor of the trap. The quality of the cap rock determines the distribution of the petroleum. The reservoirs which have good petroleum showing are always under excellent cap rocks. The scale of the petroleum is controlled by the physical property. The region, where the sandstones have fine connectivity and the reservoirs have good property will develop large scale petroleum reservoirs.Major innovation points of this thesis:(1) Making use of single hydrocarbon inclusion PVTx-t constrain method, the thesis will give the petroleum charging episodes, accumulation phases and the accumulation ages of the Silurian reservoirs of the different structural zones in Tarim basin, and then provide evidence for the heterogeneous petroleum distribution due to the varied structural evolution and varied charging history in different blocks.(2) Taking advantage of the integrated technology of micro cool/heat stage, micro fluorescent spectrum and cool cathodoluminescence, the thesis will discuss the cause and the correlation of the bitumen, free oil and oil inclusion in the clastic reservoirs. Then combined with the core observation and dissection of typical petroleum reservoirs on the macro scale, we will discuss the complex petroleum accumulation mechanism of multi-sources, multi-charging phases, adjustments and reconstructions. Eventually the thesis will lay the foundation for the determination of the dominant factors controlling petroleum accumulation and the establishment of the petroleum accumulation model in the research area.