Exploration Of Mesozoic Subtle Reservoirs In Zhen-Jing Region, Southwest Ordos Basin

With the deepening of economic reforms and opening up, China need more and more petroleum and natural gas. China, as the second largest oil importer around the world, is currently facing a sharp conflict between the supply and demand, greatly concerned about the national security of natural energy. Since the targets of exploration are gradually transferring to subtle reservoirs, relevant working methods and acknowledge systems have been shaped up. Take, for instance,"Slope break controlled reservoir","Compound accumulation of oil-gas"and"facies-eneygy coupling controlled reservoir"as examples, implausibly great achievements have been accomplished upon the practices. However, the macro rules of distribution for subtle reservoirs are yet to be aware, and the rate of exploration accuracy is supposed to be enhanced in order to meet the industrial need, both internally and externally.This paper has mainly focused on the combination of Yanchang and Yan'an Formation, developed in Mesozoic, located in Zhen-Jing Region, south west of Ordos Basin. Related data are gathered and collected from the field outcrops, drilling records, loggings, and laboratory analysis, used to identify the various sequence boundaries in the working area. Supported by the figures mentioned above, a precise analysis about the sequence frame of southwest Ordos Basin has presented a database, which includes sedimentary microfacies, information of potential reservoirs. Though the procedures of core analysis, together with the assessment on sedimentary regimes, fabric and evolution, outcomes showed further details about the distribution of sedimentary facies in the sequence framework. After the assessment of petrologic characteristics, diagenesis, reservoirs property, types and structures of porosity, a"triaxial reserch"- structural line, boundary of pinchout and overlap-as well as a"trihedron research"– plane of unconformity, bottom configuration and slope belt– defined and discussed the main characterization and contribution to the potential reservoirs. Finally, the conclusion came with the production test demonstrated that the works related to reservoir types and accumulation principles had enormously help with discover of subtle oil field in China by terms of providing creative exploration theories and techniques. Followed are the three innovative notions about this issue.1. Palaeogeomorphic Reconstruct Based on Analysis of SequencePalaeogeomorphic geography of Pre- Jurassic not only played a significant role on the early deposition pattern of Yan'an Formation, but also on the structure of features, as picturing the palaeogeomorphic geography is kind of important in exploration of subtle reservoirs. Compared with other techniques of reconstructed modeling, a raise of conception about"Maximum flooding surface"makes this paper distinguished. It is much easier to identify the boundary of unconformity with high accuracy rate by taking advantage of basic principles of sequence stratigraphy, especially practical for sections with discontinuously changes. According to the results of palaeogeomorphic reconstruction, the working area experienced an onlap from the southeast to northwest. There is utterly no Yan 10 Formation found as an outcrop-evidence, whilst partly Yan-9 Formation could be seen. By contrast, Chang 3 formation can be seen in the Zhenyuan field, and transit to Chang4+5 or Chang6 formation from northwest to southeast. Chang 6 formation emerged in Chuankou field and Qingxi paleochannel. So, result changed some strata comprehension of this field.2. Weathering Crust Modeling of Clastic RockThe weathering crust of Yanchang Formation characters itself typically in Chang 6 formation, Chuankou area, with a combination of both hydrolyzing belt and leached zone developed right under the unconformity. Hydrolyzing belt has a steady thickness with component of mudstone, spanning widely, is identified by the acceleration of drilling rate, as well as the enlarging CAL logging and the lower resistivity logging.When things come to the leached zone, unconsolidated formation with highly secondary porosity is the good feature, showing an irregular CAL of logging-data, and jump between acoustic responses. There is a great amount of alteration of kaolinite and illite though the optical screening, however, a much lower proportion of feldspar compared with different sand bodies of the formation. Regarding to the determination of trace elements-poor in K, Na, Ca and Mg, rich in Al, Si and Fe-the index of alteration is comparatively low. The on average thickness of estimation fluctuates between 0~60m.3.A comprehensive set of exploration skills for reservoirs formed by tight and low permeable sandstoneSweat spots are the critical targets in Chang-8 reservoir, which was developed beneath the source rocks. And regarding to the sequence stratigraphy analysis, together with the correspondent sedimentary facies, a form of sheet sandstones over the mouth bar, developing in the mid part of rising semicycle in MSC7, is supposed to be the target stratum. Researches illustrate that, wherever the bottom configuration is comparatively higher, sweat spots are more likely to be found. This may partially be caused by the weak development of calcite cement in Zhangjiatan shales. However, once a progress could be made in this area, exploration track goes with the same direction of foundational faults is more hopeful. Hence, we can define"clarification on the strata sequence and high spots, tracking the fractures and sedimentary facies"as the working clues in Chang-8 reserviors.Whereas the Chang-6 reservoirs lying above, sweat spots are playing an analogous role. The estimated targets are sandstones developed in distributary channels, formed in the mid-late part of a dropping-semicycle in MSC13 and MSC14. Additionally, better characteristics would like to be found in palaeogeomorphic slopes, which are crucially influenced by eluviation and diagenesis system. So, the sequence of strata, geographic location of slope and fractures are the keys of Chang-6 reservoirs.When things come to reservoirs in Yan'an Formation, given to the better reservoir quality and high separation rate, more consideration are expected for different types of the potential traps. And traps in Yan'an Formation were significantly reconstructed by differential compaction, forming a variety of depositional traps. After successful exploration, the water-oil interface could be easily confirmed. Thus is to say, the transition, traps, high point and channels concern deeply.This paper will focus on an ultimate combination of Yanchang formation and Yan'an formation in Zhen-Jing area, conclude the types of Mesozoic reservoirs and principles of distribution, and figure out the important controlling factors, in order to eventually make a precise prediction on potential targets. A set of theories aimed at the exploration for Mesozoic reservoirs are also raised, playing an informative role in the southwest Ordos Basin.