Reservoir Characteristics And Hydrocarbon Accumulation Rules Of Yan-chang Formation In Zi-chang Oil Field

Zi-chang Oil field is located in Zi-chang County, Shaanxi Province, currently the major development blocks are Ya-ping, An-ding, Yu-jiaping, Jiang-gou, Shi-jialaozhuang, Lao-caowan, Yang-jiayuanze. Producing positions of each block there are different, moreover the research work are not in the same extent, which makes it difficult to the overall planning and efficient development of oil field. For the first time, Zi-chang Oil field is seen as a whole part, to study the characteristics of hydrocarbon migration and accumulation, therefore, it has great and practical significance to the future hydrocarbon exploration and development. reducing exploration and development risk, and increasing development efficiency.Taking the disciplines and theories of petroleum geology, sedimentology, lithofacies paleography, petrology and geophysics as the guideline, and on the basis of abundant data and materials from drilling, logging, core, testing, producing analysis and research reports, this paper comprehensively studies the structural feature, sedimentary facies, reservoir rock,reservoir type and characteristics, timing and model of hydrocarbon migration and accumulation, hydrocarbon accumulation rules.The results show that the study area is a broad west-dip monocline, with nose like structure locally. the deposition system of Chang-6 member is the delta front-delta plain sedimentary system, and Chang-2 member is the delta plain sedimentary system. Underwater distributary channel and distributary channel distributes NE-SW banded. Rock types of Chang-6 and Chang-2 reservoirs are mainly gray, light gray, gray green fine-grained or medium-fine grained feldspathic sandstone. The main diagenesis of reservoir sandstone are compaction, pressolution, cementation, replacement and dissolution,etc, and it has reached the middle diagenesis A period; the pore types are mainly residual intergranular and dissoluted ones, which belongs to low porosity-extra-special low permeability～extra low porosity-ultra-low-permeability reservoir. Oil of Yan-chang formation here is the conventional oil from terrestrial facies with low density, low viscosity, low freezing point and low sulfur content. Chang-6 reservoir is the typical lithologic trap, the play type of which is reservoir-up and source-down; Chang-2 reservoir is the structural lithologic trap, the play type of which is also reservoir-up and source-down. There are two oil-gas accumulation models: one is the sandstone lens reservoir, the other is the sandstone updipping wedge-out reservoir.Taking the burial and geothermal history and illitic K-Ar isotope dating of the sandstone reservoir of Yanchang formation into account, late Early Cretaceous is fixed to be the main hydrocarbon accumulation period in Zichang, Ordos basin.Through detailed analysis of the hydrocarbon accumulation elements, the final result shows that the accumulation and distribution of hydrocarbon in Yan-chang reservoir in Zi-chang Oil field is controlled by sedimentary facies, high-quality reservoir, structural nose, regional cap rock, migration pathway, and so on. The favorable hydrocarbon accumulation zones are the place where each factors match well.