Successive formation of secondary pores via feldspar dissolution in deeply buried feldspar-rich clastic reservoirs in typical petroliferous basins and its petroleum geological significance

Clastic rock reservoirs in petroliferous basins are generally rich in feldspars. Feldspar dissolution has developed widely in clastic reservoirs, and the resulting secondary pores are crucial in deeply buried reservoirs. Based on a study of the diagenesis of clastic reservoirs in the Bohai Bay Basin, Tarim Basin, and Pearl River Mouth Basin and physical and numerical simulation experiments of fluid-rock interactions, this paper proposed a successive formation model of secondary pores via feldspar dissolution in deeply buried clastic reservoirs, considering the global research progresses in feldspar dissolution in clastic rocks. Feldspar dissolution can occur from shallow open systems to deep-ultra deep closed systems in petroliferous basins, resulting in the successive formation of secondary pores at different diagenetic stages. The successive mechanism includes three aspects. The first aspect is the succession of corrosive fluids that dissolve minerals. Meteoric freshwater dominates at the Earth’s surface and the early diagenetic A stage. Subsequently, organic acids and COformed via kerogen maturation dominate at the early diagenetic B stage to the middle diagenetic stage. COand organic acids formed via hydrocarbon oxidation in hydrocarbon reservoirs dominate at the middle diagenetic B stage to the late diagenetic stage. The second aspect is the successive formation processes of secondary pores via feldspar dissolution. Large-scale feldspar secondary pores identified in deep reservoirs include secondary pores formed at shallow-medium depths that are subsequently preserved into deep layers, as well as secondary pores formed at deep depths. Existing secondary pores in deeply buried reservoirs are the superposition of successively feldspar dissolution caused by different acids at different stages. The third aspect is a successive change in the feldspar alteration pathways and porosity enhancement/preservation effect. Open to semi-open diagenetic systems are developed from the Earth’s surface to the early diagenetic stage, and feldspar dissolution forms enhanced secondary pores. Nearly closed to closed diagenetic systems develop in the middle to late diagenetic stages, and feldspar dissolution forms redistributional secondary pores. The associated cementation causes compression resistance of the rock, which is favorable for the preservation of secondary pores in deep layers. These new insights extend the formation window of secondary pores in petroliferous basins from the traditional acid-oil generation window to a high-temperature gas generation window after hydrocarbon charging. The proposed model explains the genesis of deep-ultra deep high-quality reservoirs with low-permeability, medium-porosity and dominating feldspar secondary pores, which is significant for hydrocarbon exploration in deep to ultra-deep layers.

Mechanism of Secondary Pore Formation and Prediction of Favorable Reservoir of Paleogene in Jiyang Sag, Eastern China

Jiyang （济阳） sag is an oil rich basin, consisting of Huimin （惠民）, Dongying （东营）, Zhanhua （沾化）, and Chezhen （车镇） depressions. The clastic rock of Paleogene has undergone early and middle diagenetic stages and now the main clastic reservoir is in the middle diagenetic stage. Primary and secondary pores are developed in Paleogene sandstone, the latter is generated from the dissolution of feldspar and calcite cement in rocks owing to the organic acid from the maturated source rock, but the materials dissolved are different in different depressions. The reservoir secondary pores of Dongying depression are generated from the dissolution of calcite cement, the ones of Zhanhua and Huimin depressions from the dissolution of feldspar, the secondary pores of Chezhen depression from the dissolution of feldspar in upper section, and the dissolution of calcite cement in the lower section of Paleogene, respectively. The secondary pores are developed in two depths and the depth goes down from west to east, from south to north in Jiyang sag. The major controlling factors for secondary pore development are maturity and location of source rock. Lastly, the favorable reservoirs are evaluated according to reservoir buried depth, sedimentation, and diagenesis. The reservoir with high quality is located in the northern and central parts in Dongying depression; there are some good reservoirs in Gudao （孤岛）, Gudong （孤东）, and Gunan （孤南） areas in Zhanhua depression, and the favorable reservoirs are located in the north steep slope and the south gentle slope of Chezhen depression and central uplift, south gentle slope of Huimin depression.

Evolution characteristics and exploration targets of Permian clastic rock reservoirs in Bohai Bay Basin,East China

Based on core observation, thin section examination, fluid inclusions analysis, carbon and oxygen isotopic composition analysis, and other approaches, the structural and burial evolution histories were investigated, and the diagenetic evolution process and genetic/development models were systematically discussed of the Upper Paleozoic Permian clastic rock reservoirs in the Bohai Bay Basin, East China. The Bohai Bay Basin underwent three stages of burial and two stages of uplifting in the Upper Paleozoic. Consequently, three stages of acid dissolution generated by the thermal evolution of kerogen, and two stages of meteoric freshwater leaching occurred. Dissolution in deeply buried, nearly closed diagenetic system was associated with the precipitation of authigenic clay and quartz, leading to a limited increase in storage space. Different structural uplifting–subsidence processes of tectonic zones resulted in varying diagenetic–reservoir-forming processes of the Permian clastic reservoirs. Three genetic models of reservoirs are recognized. The Model I reservoirs with pores formed in shallow strata and buried in shallow to medium strata underwent two stages of exposure to long-term open environment and two stages of meteoric freshwater leaching to enhance pores near the surface, and were shallowly buried in the late stage, exhibiting the dominance of secondary pores and the best physical properties. The Model Ⅱ reservoirs with pores formed in shallow strata and preserved due to modification after deep burial experienced an early exposure-open to late burial-closed environment, where pore types were modified due to dissolution, exhibiting the dominance of numerous secondary solution pores in feldspar and the physical properties inferior to Model I. The Model Ⅲ reservoirs with pores formed after being regulated after multiple periods of burial and dissolution experienced a dissolution of acidic fluids of organic origin under a near-closed to closed environment, exhibiting the dominance of intercrystalline micropores in kaolinite and the poorest physical properties. The target reservoirs lied in the waterflood area in the geological period of meteoric freshwater leaching, and are now the Model Ⅱ deep reservoirs in the slope zone–depression zone. They are determined as favorable options for subsequent exploration.

Diagenesis and Its Effect on Reservoir Quality of Silurian Sandstones,Tabei Area,Tarim Basin,China

The diagenetic processes of the Tabei sandstones in the Tarim Basin include compaction, cementation （quartz overgrowths, calcite, clay minerals and a minor amount of pyrite）, and dissolution of the feldspar and calcite cement. Porosity was reduced by compaction from an assumed original 40% to about 22.1%. Cementation reduced porosity to 26.6%. The Tabei sandstones lost a little more porosity by compaction than by cementation. Quartz cementation, especially syntaxial quartz overgrowth, is a major cause of porosity-loss in many reservoirs in moderately to deeply buried sandstone. Calcite cementation played a key role in the porosity evolution of sandstones. At the early stage of burial, the early calcite cement occupied most of the pore spaces resulting in significant porosity. On the other hand, some primary porosity has been preserved due to incomplete filling or the presence of scattered patches of calcite cement. In addition to calcite, several clay minerals, including illite and chlorite occurred as pore-filling and pore-lining cements. The pore-lining chlorite may have helped in retaining the porosity by preventing the precipitation of syntaxial quartz overgrowths. Illite, which largely occurred as hair-like rims around the grains and bridges on the pore throats, caused a substantial deterioration of penetrability of the reservoir. Calcite cement dissolution was extensive and contributed significantly to the development of secondary porosity.

Research on Petroleum Reservoir Diagenesis and Damage Using EDS Quantitative Analysis Method with Standard Samples

In recent years, the X\|ray spectrometer has been developed not only just in enhancing resolution, but also towards dynamic analysis, computer modeling processing, sampled quantitative analysis and supra\|light element analysis. With the gradual sophistication of the quantitative analysis system software, the rationality and accuracy of the established sample deferential document have become the most important guarantee to the reliability of sample quantitative analysis. This work is an important technical subject in China Petroleum Reservoir Research. Through two years of research and experimental work, the EDS quantitative analysis method for petroleum geology and reservoir research has been established, and referential documents for five mineral (silicate, etc.) specimen standards have been compiled. Closely combining the shape characters and compositional characters of the minerals together and applying them into reservoir diagenetic research and prevention of oil formations from damage, we have obtained obvious geological effects.

Properties and Characterization of Modified HZSM-5 Zeolites

Physicochemical and catalytic properties of phosphorus and boron modified HZSM-5 zeolites treated with 100% steam at 673 K were investigated. The acidity and distribution of acidic sites were studied by infrared spectroscopy using pyridine as probe molecule and temperature programmed desorption (TPD) of ammonia. The structure of the samples was characterized by XRD, and the textural properties of the catalysts were determined by nitrogen isothermal adsorption-desorption measurements and scanning electron microscopy (SEM). The XRD results show that the modified samples have no novel crystalline phase, indicating a high dispersion of phosphorus and boron species. After treatment, the microporous volume and surface area of the samples markedly decrease, implying the blockage of the channel. The nitrogen adsorption-desorption measurements suggest that the isothermal type of all samples is a combination of isothermal type I and IV, and all hysteresis loops resemble the H4-type in the IUPAC classification. The total acidity of the modified samples, determined by pyridine adsorption IR and TPD of ammonia, decreases in contrast to that of the parent HZSM-5. The conversion of n-heptane over P and B steam-modified HZSM-5 is higher than that of P and B-modified HZSM-5 zeolites but lower than that of the parent HZSM-5.

Reservoir characteristics of the first member of Middle Permian Maokou Formation in Sichuan Basin and its periphery and inspirations to petroleum exploration,SW China

Based on a large number of field outcrops and cores taken systematically from boreholes,by microscopic observa-tion,physical property analysis,mineralogy analysis,geochemical analysis etc.,reservoir characteristics of the first member of Middle Permian Maokou Formation in Sichuan Basin("Mao 1 Me mber"for short)are analyzed.(1)Rhythmic limestone-marl reservoirs of this member mostly exist in marl layers are a set of tight carbonate fracture-pore type reservoir with low porosity and low permeability,with multiple types of storage space,mainly secondary dissolution pores and fissures of clay minerals.(2)The clay minerals are mainly diagenetic clay minerals,such as sepiolite,talc and their intermediate products,aliettite,with hardly terrigenous clay minerals,and the reservoir in different regions have significant differences in the types of clay minerals.(3)The formation of high quality tight carbonate reservoir with limestone-marl interbeds is related to the differential diagene-sis in the early seawater burial stage and the exposure karstification in the early diagenetic stage.It is inferred through th e study that the inner ramp of southwestern Sichuan Basin is more likely to have sweet spots with high production,while the outer ramp in eastern Sichuan Basin is more likely to have large scale contiguous reservoir with low production.

Reservoir formation mechanism analysis and deep high-quality reservoir prediction in Yingcheng Formation in Longfengshan area of Songliao Basin, China

Although commercial gas flow was produced in several wells with recent years'exploration of Longfengshan area in Changling fault sag,the formation mechanism and controlling factors for high-quality reservoirs still remained undefined.Here,the Yingcheng tight gas reservoirs of Longfengshan area are used as an example to characterize high-quality reservoir formation mechanism and distribution rules.Based on the thin section,SEM,X-ray diffraction,computed tomography(CT)scanning,burial history,constant-rate mercury penetration and physical properties testing,formation mechanism and controlling factors for high-quality reservoirs were analyzed.Results show the following characteristics.First,the reservoir is dominated by chlorite and laumontite cements,and compaction is the most important factor to control reservoir physical properties.According to this,the reservoir can be divided into compacted tight sandstones,chlorite-cemented sandstones and laumontite-cemented sandstones.Second,the high-quality reservoirs are formed due to early extensive laumontite precipitation and the later dissolution of laumontite by organic acid.Meanwhile,it is found that the distribution of cementation and dissolution exhibits some regulations in sedimentary facies,and the distribution is mainly effected and controlled by the lake water and charging of fresh water.Besides,the distribution model of various types of sandstones was established.Studies over diagenesis and sedimentary facies reveal that the high-quality laumontite-cemented sandstones exist in the outside subaqueous fan-delta of the deep sag in Longfengshan area.These findings have been validated by recent exploration wells which obtained high industrial gas flow.