Reservoir Evaluation And Natural Gas' Enrichment Of Ma5 Member In The Eastern Jingbian Buried Platform

This dissertation studies synthetically the reservoir characteristics and natural gas' enrichment at the top of fifth member of The Ordovician Majiagou carbonate formation of Jingbian Gas-field, Ordos Basin. Drawing on the findings of previous research on this topic, comprehensively applying the theories of relevant disciplines such as petrology, sedimentology, structural geology, petroleum geology, oil and gas field development geology, and geostatistics, and analyzing the data collected from core observation, rock casting thin sections, log, physical properties, high-pressure mercury, scanning electron microscopy and cathodoluminescence analysis, and gas well test of Shaan 200 well area, this study arrives at the following conclusions in the hope of providing theoretical guidelines and development suggestions for the eastward expansion of the Jingbian Gas-field.(1) The study area is relatively stable in tectonic environment, with some inherited similarities in the uplift and depression of the upper layers of the fifth member of Majiagou formation. The two nose-shaped upheavals, which have developed from west to east in the study area may somewhat exert some control over the enrichment of natural gas. The three ancient valley, which have developed in the study area, play an important role in controlling the degree of natural gas enrichment of the fifth member of Majiagou formation.(2) The depositional environment of the study area is characterized primarily by the extensively developed dolomite fiat microfacies, and secondarily by gypsum-bearing dolomite flat microfacies and gypsum-dolomite flat microfacies. Different types of sedimentary microfacies and diagenesis of the study area have created the reservoir space made up of primarily intercrystalline dissolution pores and intercrystalline pores, and secondarily solution vugs and anhydrite-moldic pores. The extensively developed micro-fissures link up various types of pores, serving as the main channels for natural gas to migrate vertically in the trenches and then laterally in the reservoir. For the upper layers of the fifth member of Majiagou formation, the better configuration of trenches and micro-fissures, namely, the better configuration of vertical and lateral channels, the higher degree of natural gas enrichment.(3) On the basis of heterogeneity formed by deposition and diagenesis. the degree of micro-fissure development plays an important role in controlling heterogeneity. In the micro-fracture development zone, micro-fissures can improve the connectivity among intercrystalline dissolution pores, intercrystalline pores, dissolution pores (vugs) and anhydrite- moldic pores, etc. In particular, the microfissure-intensive development area filled with a large number of dissolution pores (vugs) is usually the formation of high-yield gas reservoir. On the other hand, this situation will increase the non-homogeneous difference between the microfissure -developing and non- microfissure -developing zones, leading to the significant heterogeneity in distribution area of gas reservoirs and enrichment of natural gas in the study area.(4) The reservoir space of the study area is divided into four types of pore combination, with the type of microfissure - dissolution pore (vug) - pore combination as the leading one. A comprehensive analysis shows that the main reservoirs in the eastern part of the gas field are still Ma₅(1-3), Ma₅(1-4) and Ma₅(2-2). The performance of Ma₅(4-1)is poor and it cannot be a main reservoirs in the eastern part of the gas field. By the criteria for reservoir classification, the main reservoirs in the study area are Category II and Category III, which are of relatively stable thickness and continuous development. These two categories of reservoirs has such strong points as good configuration of pores and microfissure, good performance of reservoir and seepage, and a sizable gas-bearing area. However, their heterogeneity should be taken seriously in the process of exploration and development.