Low permeability tight gas resources account for 90% of the Xihu Sag. Under the background of extensive development of low permeability and tight reservoirs, the key to economic and effective development is to find sweet reservoir formation. To clarify the origin and distribution of a sweet reservoir in the study area, it is important to study the formation and evolution mechanisms of chlorite. In this study, based on the analysis of thin section, X-ray diffraction and SEM, through the analysis of the key factors in the formation of authigenic chlorite of the Huagang Formation in the middle and north of the central inversion structural belt, we reasoned the formation and evolution process of chlorite in the whole life cycle. According to the sedimentary diagenetic response characteristics of chlorite, two types of favorable sedimentary facies belts of chlorite are identified. The results showed that the development of pore-lined chlorite is a natural advantage of reservoirs in the East China Sea. Chlorite is formed under the joint action of three factors: the source of iron and magnesium ions, the alkaline environment in the early diagenetic stage and the open fluid field. After the formation of pore-lined chlorite, the sweet spots developed under the protection of four mechanisms: inhibiting quartz cementation, enhancing compression resistance, protecting macropore throat and primary pores, and promoting secondary intergranular dissolved pores. When the content of chlorite in the pore lining is high (relative content > 35%), the lining thickness is moderate (4–10 μm). A high degree of wrapping and good crystallization are conducive to the formation and preservation of sweet spots.
A new modeling method is proposed to address the shortcomings of conventional geological modeling methods in simultaneously characterizing the geometry, stacking patterns, and well data of the channel bar in braided river. The channel bar model is reconstructed based on the target body, which is defined by the channel bar’s geometric parameters points, lines, and surfaces. In the simulation process, the distributions of sandstone and mudstone are simulated first. Then, the channel bars at the well are simulated based on the channel bars identified on the well and the thickness of the sandstone, the priori geometries, and sizes of the channel bars. It is judged whether the proportion of the simulated channel bars has reached the global proportion. If not, a randomly selected point between wells is used as a virtual well, and the sandstone thickness is used to determine whether it is a single or composite channel bar. The process is repeated until the proportion of channel bars reaches the set proportion. Take the N gas field as an example. The analysis of the geometry, variance function, thickness distribution histogram, and volume distribution histogram of the simulation results shows that the new method can better characterize the channel bar’s geometry, sizes, and stacking patterns than the multipoint geostatistical modeling method Snesim.
Generally, the favorable diagenetic facies belt of the reservoirs is the dessert for hydrocarbon exploration. Traditionally, the research of diagenetic facies is based on core analysis, as a result, it is difficult to make prediction under the condition of sparse wells and less cores. In order to solve this difficulty, the new method to predict the diagenetic facies should be researched. In this research, the low permeability sandstone reservoirs of East China Offshore Gas Fields are selected as the research area, and three research steps have been done. First, by using the core analysis, ACR (Apparent Compaction Ratio) and ADR (Apparent Dissolution Ratio) are selected as the discriminant parameters for the diagenesis, and the quantitative standards are established. Second, the rock-electric relationships according to the diagenetic facies are established, the relationship of well logging parameters (GR, DT, CNCF, RT, DEN) vs. ACR, and the same well logging parameters vs. ADR can be matched by using BP neural network method. Accordingly, the ACR and ADR of the whole well can be obtained, and the diagenetic facies in accordance with the ACR and ADR can be predicted. Third, the relationship of well-log and seismic data can be established. After that, the relationship of seismic attributes (Ip - p wave impedance, Is - s wave impedance, Vp/Vs - p and s wave velocity ratio) vs. ACR, and the same seismic attributes vs. ADR can be matched. After the three steps, the favorable diagenetic facies belt can be predicted by using seismic attributes under the condition of sparse wells and less cores. By using the above methods, the diagenetic facies in the research area can be divided into six types: mid dissolution and mid compaction; mid dissolution and mid-strong compaction; strong dissolution and strong compaction; mid-strong dissolution and strong compaction; shale-silty strong compaction; strong cementation. The previous three of the six types are selected as the favorable diagenetic facies. According to the quantitative standards of ACR & ADR, and the relationship of the seismic attributes vs. ACR & ADR, the diagenetic facies in the plane and vertical can be predicted by using seismic attributes. In the research area, three layers of Huagang formation have been selected by fitting the favorable diagenetic facies: Layer 1 (Shallow) corresponding to mid dissolution and mid compaction; Layer 2 (Mid) corresponding to mid dissolution and mid-strong compaction; Layer 3 (Deep) corresponding to strong dissolution and strong compaction. As a result, these three layers are considered to be the desserts for exploration and development. This paper presents a new quantitative characterizing system for diagenetic facies by combing core-logging-seismic. Furthermore, a new method to quantitative characterizing diagenetic facies by using seismic attributes is established for the first time. The innovative content presented in this paper can provide theoretical references for exploration of sandstone reservoirs under the condition of sparse wells, and also provide complement for the diagenetic facies characterizing theories.
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