Research On Sedimentary Facies And Microcharacteristics Of Yanchang Formation Of Northern Yaodian Oilfield In Northern Shaanxi

Ordos Basin is the second largest inland sedimentation basin with prolific in oil, gas, coal and uranium resources. After years of exploration and research, many lithology reservoirs were discovered, it is indicated that the Mesozoic Erathem formation of Ordos Basin will likely continue to yield additional oil and gas discoveries for many years to come, that is of great practical and economic significance to the exploration and exploitation of hydrocarbon resources.Northern Yaodian Oilfield lies on eastern part of Shanbei Slope in Ordos Basin and the primary oil-bearing bed is Yanchang Formation. It is implied that there is biggish space for exploration in the future. However, our understanding of the hydrocarbon accumulation regularity of Yanchang Formation in northern Yaodian Oilfield was not deepened due to the weakness researches of sedimentary facies, the characteristics of reservoirs and the master controlling factors of oil accumulation, a momentous breakthrough in oil exploration has not been made in past years. In allusion to those issues about sedimentary facies, microcharacteristics of sandstone reservoirs and hydrocarbon enrichment rule, the much systemic researches have been carried out by means of sedimentology, reservoir sedimentology, sequence stratigraphy, petroleum geology and analyzing, testing measures.Yanchang Formation is divided into four scale Cycles, that is 1 Super-long-term Sea Level Cycle, 4 Long-term Sea Level Cycles, 9 Middle-term Sea Level Cycles and 24 Short-term Sea Level Cycles. They are compared with traditional stratigraphic classification. Based on analyzing palaeocurrent direction, light minerals, heavy minerals and REE characteristics combined with the regional geologic background, it is suggested that sediments in this area originate from the northern part of Ordos Basin in the period of Yanchang Formation. In addition, it is indicated that the parent rocks are metamorphic rocks and magmatic rocks in Yinshan mountain at the northeast side of Ordos basin. Based on source direction, depositional characteristics and studying in the fields, delta facies and fluvial facies are identified, of which distributary channels in delta plain microfacies and braided channels microfacies controlled the distribution of sand bodies and accumulation of oil and gas.It was shown that the sandstones are medium to granule arkose, which the mud matrix is widespread but not uniform distribution and the type of cements observed including calcite, quartz and feldspar overgrowths, clay minerals and laumonite. The main clastic particles constituents observed in sandstones are middling to fine in separation, sub-angular and sub-rounded but moderately to well sorted. These grains mainly have point-line or lines contact, membrane-porosity typed cementation. It was also shown that the main types of pores are remnant intergranular pores and dissolved pores. The reservoirs are poor in pore throat distribution, complex in structure, small and asymmetry in throat showing heterogeneity. But relatively speaking, the quality of Chang 2 sandstone reservoirs is the best and Chang 6 is the second while they are better than others. Taken as a whole, they are the typical low porosity and permeable reservoirs.It is shown that the diagenesis of the sandstones mainly include compaction, cementation and dissolution, which are in the late diagenesis phase A while part of the Chang 6 sandstones are in the A to B stage of late diagenesis. At the same time, the compaction and cementation destroyed their primary pore intensively, dissolution changed the pore structure effectively, consequently the reservoirs quality was improved. The diagenetic facies include compaction and pressolution facies, weak compaction-chlorite film cementation facies, compaction-quarts and feldspar overgrowth cementation facies, compaction-carbonate cementation facies, mud-cementation facies, and laumonite-feldspar dissolution facies, and favourable reserviors mostly occurred in the sandbodies of weak compaction-chlorite film cementation and laumonite-feldspar dissolution diagenetic facies belts. While rest four diagenesis facies developing zones have weak properties of matter and always turned into diagenetic tight zones. Besides the diagenesis, deposition is another important factor to the reservoirs quality. Integrated these researches with practical situation in this area, the sandstone reservoirs are divided into four types. I -good reservoirs are occur in the Chang 2 braided river channel sandbars, II -appreciably good reservoirs and Ill-poor reservoirs are occur in the Chang 6 delta plain distributary channels.According to the study of the geological conditions for oil and gas accumulation, sedimentation, tectonism and diagenesis not only are basement of hydrocarbon accumulation, but also control its enrichment and distribution. On the base of range of profitability, some advantageous blocks of oil are predicted of Chang 6 and Chang 2 Member in northern Yaodian Oilfield, which provide the credibility geology gist for further exploration and exploitation in this area.