| Low permeability and low porosity (lower permeability and lower porosity) oil gas reservoir accounts for a rather high proportion pf domestic oil gas reservoir. However, there has been not yet a suit of perfect theoretic and techniques about how to exploit the low permeability and low porosity oil gas reservoir at present. The gas field in northeast of Sichuan is a typical low permeability and low porosity gas reservoir, and at present the specific geological characteristics for which is still not clear.Because of the Xujiahe formation of Yuanba area buried deep, diagenesis strong and under the long-term influence of extrusion; it reservoir is mainly of low permeability and low porosity gas reservoir. The reservoir heterogeneity strong, the reservoir property bad, and the exploration of which is difficult. So further research of the reservoir is of great significance. The research take sedimentation, reservoir geology, logging geology and mineral petrology, geochemistry and reservoir comprehensive evaluation and other theory method as guidance. In combination with regional geological background, the tectonic evolution, from the core, logging and analysis of test data, uses various means for the fine description of Xujiahe low permeability and low porosity reservoirs of the petrology characteristics, diagenesis, reservoir porosity evolution and other characteristics. Made the following main achievements:Has a clear recognition that the research area mainly for braided fluvial river delta depositional. The reservoir sand body with better property is mainly developed in high quality phase belt. And we focus on the analysis of the main control factors of research area low permeability and low porosity reservoirs and cause mechanism and evolution rule of the pore, and then make comprehensive assessment to the purpose layer with the chosen evaluation parameters of the low permeability and low porosity of reservoir in the study area.1. The study shown that Xujiahe formation mainly developed two kinds of reservoirs:one is quartz sandstone reservoirs (developed in the middle mild-segment of member2) and calcium flocks sandstone reservoirs (developed in the member3) mainly for crack-residual hole. The reasons for the formation of crack are relate to regional tectonic movement and the local structural deformation; the main reason for the formation of quartz sandstone reservoirs’ porosity was compressive action of a large number of quartz grain skeleton results in the preservation of the primary pores. The main reason for the formation of calcium flocks sandstone reservoirs porosity was the formation of high temperature gradient, local existed abnormal high pressure make the preservation of the primary pores; The other one is lithic sandstone reservoirs which characterized mainly by dissolution pores (do not exclude the existence of a few primary pores). The dissolution mechanism is components can be dissolute in reservoir are corroded. This kind of reservoirs mainly develops in member2and lower mild-segment of member4, the development degree depends on the content of soluble composition and the content of carbonate cement.2. There are two types of dominating fissures in the research area:one is dominated by structural fractures. From the core observation, there are multiple of group cracks and parallel to one and another. As for the reasons:the main basins of the research area are foreland basins, developed multi-period foreland thrust structure. Rocks are broken down under the tectonic stress action and formed lots of parallel structural fractures. The other one is dominated by digenetic fracture:Large numbers of petrographic thin section observation shown that around the major fracture exist interfingering microfracture, some fractures even corrode into snake shape along the edge of the particles (dissolution seam) or form microfracture along the edge of thicker particles (grain boundary fracture). The pore types of the research area mainly divided into two kinds, one is intergranular pore, which mainly consist of residual intergranular pore and micro hole formed by the corrosion of matrix in this area. Residual intergranular pore mainly develop in quartz sandstone reservoirs. A lot of residual intergranular can be preserved under the handling and washing function of tidal or waves. Matrix pores mainly developed in lithic sandstone reservoirs, the main reasons for the formation of which is acidic fluid dissolution.The other one is intragranular pores, which mainly developed in lithic sandstone reservoirs. The main reason is a large number of organic acids corroded feldspars and debris when organic matters matured.3.According to logging data、property analysis data and the results of chip identification comprehensive analyzed lithofacies types and distribution of the research area. Favorable reservoir development lithogenous phase is shale package dissolution diagenetic phase, mainly developed in lower mild-segment of member2. Dissolution diagenetic phase mainly developed in lower mild-segment of member4, silicon cementation burst phase mainly developed in middle mild-segment of member2,calcium cementation rupture dissolution phase mainly developed in member3; Adverse development of the reservoir lithogenous phase is strong compaction-phase,mainly developed in lower mild-segment of member2with higher VSH and higher plastic debris content. Calcium cementation phase mainly developed in upper mild-segment of member2and upper mild-segment of member4. Through the coring dates established diagenetic phase standards, rise to logging mode and divide the diagenetic phase type without sampling location. Then, provide favorable basis for plane distribution of study area.4.The development of reservoir is controlled by such factors as sequence、 deposition、diagenesis and structure. Research shows that sedimentary facies belt is the material base which controls the evolution of reservoir sand body’s pore. Sedimentation controls the sand body’s type、shape、thickness、and scale on macroscopic, besides, it’s also controls the size of clastic particles、the amount of interstitial matter、rock structure characteristics(separation、psephicity and type of contact), accordingly, controls the primitive intergranular pore and the stand or fall of permeability. Through the reservoir physical properties and sedimentary microfacies statistical analysis found that:better reservoir physical properties all develop in main stem deposition, and because clastic particles deposited in main stem are much coarse, the matrix content is low, and the stability particles content is high. Sufficient hard particle action as valid skeleton have certain compression capability, more conducive to the preservation of intergranular pore and later dissolution.Research shows that quartz sandstone reservoirs and calcium flocks sandstone reservoirs have much similar key controlling factors. Firstly, better physical property reservoir sand bodies all develop in main stem deposition, granularity of sedimentation are coarse, and carbonate cement formed in diagenetic stage are much lower. The corresponding governing factor of lithic sandstone reservoirs is much complex: sedimentation mainly embodied in granularity size and the difference of soluble mineral content. A lot of statistic data indicated:reservoirs mainly develop in thick lithic sandstone, and soluble components like feldspar metamorphic rocks and volcanic debris and the interstitial material content are positively related to particle size. The influence of diagenesis mainly embodied in compression cementation and dissolution. The compression in the research area is strong, influence range is very large; carbonate cementation is stronger topically, serious damaged the reservoir physical properties; dissolution, which range controlled by soluble components, is the main construction diagenesis and mainly developed in lower mild-segment of member2and lower mild-segment of member4.5. According to the research of sedimentary facies characteristics、reservoir characteristics and reservoir log prediction, three aspects of the reservoirs:combined with the oil and gas accumulation elements, comprehensive analysis favorable zone prediction must take the following principles into account synthetically,①sedimentary facies is the geologic foundation of favorable zones evaluation,②reservoir-cap born is the important condition of favorable zones evaluation,③diagenesis is the key factor of favorable zones evaluation,④logging prediction is the light of the favorable zones evaluation. According to the reservoir control factors, combined with sedimentary environment、mineral composition distribution characteristics and diagenesis, make favorable zones predictions for each sand group of the Xujiahe formation of Yuanba area. The results show that:lower mild-segment of member2is the main exploration sediment group of the research area. Quartz sandstone and calcium flocks sandstone have the highest potential. Member4is the most valuable exploration sediment group. |
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