| The Triassic Yanchang Formation which is characterized by low-permeability is the most important petroliferous formation in Ordos Basin. The discover of large scale low porosity and permeability sandstone reservoir which has a geological oil reserves exceeding 100 millions tons such as Xifeng, Jiyuan and Huaqing oil pools demonstrate that there are abundant hydrocarbon resource and favorable exploration prospect in Yanchang Formation. Meanwhile, the misunderstood of hydrocarbon accumulation condition and mechanisms is turning into the restrict of exploration breakthroughs in deep oil pool of Yanchang Formation. This paper focuses on Yanchang Formation in Daijiaping area Ordos Basin as the object, using the theory and method of petroleum geology, geochemistry and physical geography, and the date such as borehole, core stone, well logging and earthquake, discussion the forming process and accumulation mechanisms of Yanchang Formation low porosity and permeability sandstone in Daijiaping area, according to the analysis of hydrocarbon accumulation condition such as source rock geochemical characteristics, fracture structure characteristics and the forming process of low porosity and permeability sandstone.1.There are several sets resource rock in chang4+5-chang9 oil layer group, which belong to delta front deposition. Characterized by great thickness, high abundance of organic matter, high maturation and organic matter type is mixed of humic and sapropelic. The crude oil in Yanchang reservoir have similar geochemical character, the group composition is dominated by saturated hydrocarbon, is formed in fresh-brackish water and semi-deep lake sedimentary, and the kerogen is mixed of low aquatic organisms and terrestrial higher plant. We dividual the crude oil into two different types according to cluster analysis, there are genetic relationship between different type of crude oil and chang7 source rock base on factor analysis and multi-parameter comprehensive.2. There are large number of fault in Daijiaping area distributed in NE and NWW, which are mainly high angle thrust fault, and there development some normal fault, strike-slip fault and detachment fault as well. and extend to large scale.The NWW-trending fault are normal fault, mainly distributed in the southwest of study area, are resulted from extensional compression in Yanshanian, while the NE-trending fault are mainly high angle thrust fault, are resulted of compression in Himalaya. The are four kinds of fracture such as NE-SW NW-SE SN and NEE-SWW in Yanchang Formation, which are mainly high dip fracture and have a well vertical penetration. The fracture come out of tectonic stress field in Yanshanian and Himalaya, the NW-SE fracture are formed in NWW-SEE crushing stress in Yanshanian and the SN, NE-SE and EW fracture are formed in NNE-SSW crushing stress in Himalaya. The low-amplitude nose shaped uplift in Yanchang formation which caused by fault structure are favorable exploration target area, and the fault which formed in Yanshanian have excellent transporting ability are great pathway of oil and gas migration, meanwhile, closure of the fault enhance because of the squeezing action from basin-marginal which is favorable for hydrocarbon preservation.3、The Yanchang Formation reservoir are mainly feldspathic litharenite and lithic arkose, porosity are distribute between 4% and 15%, general less than 10%, while permeability are distribute between 0.02×10-3μm2 and 4.0×10-3μm2, general less than 1.0×10-3μm2, characterized by low permeability and extra-low permeability reservoir. The sandstone reservoir undergone intense diagenesis alteration such as compaction, cementation, metasomatism and dissolution according to conventional section, cast thin section, X-ray diffractometry and scanning election microscope observe, early compaction, carbonate cementation, quartz overgrowth and authigenic kaolinite precipitation are the main reasons causing the propensity loss, the chlorite cement, feldspar and debris secondary erosion improve the reservoir quality, intense diagenesis is the main reason of reservoir densifying. There are various types of pore and complicated pore structure in Yanchang reservoir sandstone, reservoir space of Yanchang reservoir are mainly intragranular dissolved pore and residual intergranular pore with flaky, while the throat are mainly curved lamellar throat and necking throat. The pore diameter distribute between 10μm and 80μm,which are mainly mesopore and small pore, while the throat redius distribute between 0.05μm and 1.68μm, which are mainly fine and micro throat. We dividual the reservoir into four different kinds according pore structure parameters and mercury-injection tracing pattern, in which I and II types of reservoir pore distribution homogeneous and are mainly mesopore fine throat are and small pore, III and IV types reservoir pore distribution nonuniform and are mainly small pore micro throat.4.There are different kinds of inclusion such as mainly saline inclusion and hydrocarbon inclusion in Yangchang reservoir. We dividual the inclusion into two kinds arrconding to the characteristic of inclusion occurrence, inclusion abundance, vapor-liquid ratio and homogenization temperatures of the coeval aqueous inclusion, the first phase develop in diagenesis minerals which born in early stage of diagenetic stage such as secondary enlargement of quartz and feldspar, fractures of quartz, with a low abundance, GOI of inclusion in first phase are generally less than 5%, homogenization temperatures of the coeval aqueous inclusion distribute betwee 80℃ and 95℃.The second phase are mainly develop in diagenesis minerals which born in later period of diagenetic stage such as quartz overgrowth, calcite cement in intergranular and feldspar dissolute fracture, homogenization temperatures of the coeval aqueous inclusion distribute betwee 105℃ and 120℃. The two phase accumulation process are late Jurassic and late period of early Cretaceous, the crude oil in late Jurassic hydrocarbon filling phase is low-mature oil with a smaller scale, while the large scale hydrocarbon filling in late period of early Cretaceous is normal crude oil which is the main hydrocarbon filling in Yanchang Formation.5.The porosity of Yanchang reservoir sandstone are 7.80～12.29%,while original porosity are 36.59 ～ 37.91%, the porosity after compaction are 18.25 ～ 22.74%, porosity loss cause by chlorite, quartz overgrowth and I type carbonate cement are 1.26%～4.36%, while the porosity after early stage cementation are 17.62～20.94%, base on the microscopy statistics about surface porosity of different cement, secondary erosion pore in consequence of feldspar and debris secondary erosion are 3.64%～ 5.12%, and the reservoir porosity are 18.26% ～22.09% after secondary erosion, porosity loss cause by Ⅱ type carbonate and authigenic kaolinite precipitation in late diagenesis stage cement are 1.26%～4.36%, indicate that early compaction and late carbonate cementation are the main reasons causing the proporsity loss. The porosity before the large scale hydrocarbon filling in late period of early Cretaceous are 15.49～18.16%%, permeability are 0.28～0.57x10-3μum2, manifest that the reservoir have a high porosity and permeability in late period of early Cretaceous,and have not densification at this time.6.We reestablishment the dynamic process of reservoir base on hydrocarbon accumulation process comperhensive analysis, the buried deep of Yangchang Formation attain 2200～2400m and the temperature get to 80～100℃ at late Jurassic, and clay mineral transformation at a lower level, indicate that the organic matters are low-mature and achieve hydrocarbon generation thresholds. The temperature bring down and the source roke thermal evolution break off because of strate uplift at the late phase of late Jurassic,the amount of hydrocarbon which generate at this stage is quite limited. The scale of fluid process is quite limited and the reservoir have not completely densifying. The organic acid and low mature crude oil which product by organic material splitting get in to the reservoir along the structural fracture, and generate numerous of secondary erosion pore. The burying deeping get to 2400～ 2600m with the basin subsidence in metaphase of early Cretaceous, corresponding temperature get to 90y120℃,source rock thermal evolution in mature stage and generate a large number of hydrocarbon. The formation temperature bring down along with the tectonic uplift at late Cretaceous, the medium enviromen change from faintly acid in early stage to alkalescence following the consume of reservoir organic acid, CO2 precipitate and formation II type calcite result in the destory of pore struct and loss of porosity, which is the key factor of densification in Yanchang reservoir. |
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