Episodic hydrothermal alteration on Middle Permian carbonate reservoirs and its geological significance in southwestern Sichuan Basin,SW China

To analyze the episodic alteration of Middle Permian carbonate reservoirs by complex hydrothermal fluid in southwestern Sichuan Basin,petrology,geochemistry,fluid inclusion and U-Pb dating researches are conducted.The fractures and vugs of Middle Permian Qixia–Maokou formations are filled with multi-stage medium-coarse saddle dolomites and associated hydrothermal minerals,which indicates that the early limestone/dolomite episodic alteration was caused by the large-scale,high-temperature,deep magnesium-rich brine along flowing channels such as basement faults or associated fractures under the tectonic compression and napping during the Indosinian.The time of magnesium-rich hydrothermal activity was from the Middle Triassic to the Late Triassic.The siliceous and calcite fillings were triggered by hydrothermal alteration in the Middle and Late Yanshanian Movement and Himalayan Movement.Hydrothermal dolomitization is controlled by fault,hydrothermal property,flowing channel and surrounding rock lithology,which occur as equilibrium effect of porosity and permeability.The thick massive grainstone/dolomites were mainly altered by modification such as hydrothermal dolomitization/recrystallization,brecciation and fracture-vugs filling.Early thin-medium packstones were mainly altered by dissolution and infilling of fracturing,bedding dolomitization,dissolution and associated mineral fillings.The dissolved vugs and fractures are the main reservoir space under hydrothermal conditions,and the connection of dissolved vugs and network fractures is favorable for forming high-quality dolomite reservoir.Hydrothermal dolomite reservoirs are developed within a range of 1 km near faults,with a thickness of 30–60 m.Hydrothermal dolomite reservoirs with local connected pore/vugs and fractures have exploration potential.

Reconstruction and petroleum geological significance of lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in northwestern Sichuan Basin,SW China

Based on analysis of outcrop,drilling,logging and seismic data,and geotectonic background,the lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in the northwestern Sichuan Basin were reconstructed,and the petroleum geological significance of the lithofacies paleogeography and paleokarst geomorphology were discussed.The Maokou Formation is divided into 3 long-term cycles,namely LSCl,LSC2 and LSC3,which correspond to the Member 1,Member 2 and Member 3 of the Maokou Formation,respectively.Controlled by the extensional structure caused by opening of the Mianlue Ocean in the north margin of the upper Yangtze blocks and basement faults produced by mantle plume uplifting,the area had tectonic differentiation in NWW and NE,and sedimentary basement took on episodic settlement from north to south,as a result,the sedimentary systems of Member 1 to Member 3 gradually evolved from carbonate platform to platform-slope-continental shelf.According to the residual thickness,paleokarst geomorphologic units such as karst highland,karst slope and karst depression at different stages were reconstructed.The karst geomorphological units were developed successively on the basis of sedimentary geomorphology.Sedimentary facies and paleokarst geomorphology are of great significance for oil and gas accumulation.The Maokou Formation in northwestern Sichuan has two kinds of most favorable reservoir zone combinations:high energy grain shoal and karst monadnock,platform margin slope and karst slope.Based on this understanding,the planar distribution of the two kinds of reservoir zones were predicted by overlapping the favorable reservoir facies belt with paleokarst geomorphology.The study results provide a new idea and reference for the exploration deployment of the Middle Permian Maokou Formation in the Sichuan Basin.

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.

Relationship between Permian volcanic rocks distribution and karst paleogeomorphology of Maokou Formation and its significance for petroleum exploration in western Sichuan Basin, SW China

Based on outcrop, drilling, logging, geochemical analysis and seismic data, the karst landform and distribution of Permian volcanic rocks at the end of the sedimentary period of the Maokou Formation in the western Sichuan Basin are examined, and their petroleum geological significance is discussed. Affected by normal faults formed in the early magmatic activities and extension tectonic background in the late sedimentary period of the Maokou Formation, a local karst shallow depression under the background of karst slope came up in the Jianyang area of the western Sichuan Basin, where the residual thickness of the Maokou Formation was thinner. Basic volcanic rocks like pyroclastic rock of eruptive facies, basalt of overflow facies, diabase porphyrite of intrusive facies and sedimentary tuff of volcanic sedimentary facies were formed after karstification. However, under the effects of faulting and karst paleogeomorphology, the volcanic rocks in different areas had different accumulation features. In the Jianyang area, with long eruption time, the volcanic rocks were thick and complex in lithology, and accumulated in the karst depressions. In the Zhongjiang-Santai area located in the karst slope, there’s no fault developed, only thin layers of basalt and sedimentary tuff turned up. The karst landform controls the build-up of thick explosive facies volcanic rocks and also the development of karst reservoirs in the Maokou Formation, and the western Sichuan area has oil and gas exploration potential in volcanic rocks and the Maokou Formation.

Type and distribution of Mid-Permian Maokou Formation karst reservoirs in southern Sichuan Basin,SW China

Based on the analysis of the responses of conventional logs such as natural gamma(GR), density(DEN), acoustic interval transit time(AC), compensated neutron(CNL), dual lateral resistivity(Rlld, Rlls), and caliper log(CAL), combined with drilling data,cores, thin section and productivity of 65 wells, the reservoirs in the Mid-Permian Maokou Formation of southern Sichuan Basin were divided into four types, fractured-vuggy, pore-vuggy, fractured and fractured-cavity. The main reservoirs in high productivity wells are fractured-vuggy and pore-vuggy. The reservoirs of Maokou Formation are generally thin, and can be divided into the upper reservoir segment(layer a of the second member to the third member of Maokou Formation, P_2 m^2 a-P_2 m^3) and the lower segment(layer b of the second member of Maokou Formation, P_2 m^2 b). The two reservoir segments are mainly controlled by two grain beaches during the sedimentation of P_2 m^2 a-P_2 m^3 and P_2 m^2 b, the vertical zonation of karst, and the fractures. The upper reservoir segment is generally better than the lower one in development degree and single well productivity, and is much thicker than the lower one. It is thicker in the Yibin-Zigong-Weiyuan-Dazu area, the southwestern area of Chongqing and the southeastern area of Luzhou, while the lower segment is thicker in the Neijiang-Zigong-Luzhou area and the Dazu-Luzhou area. The areas with big reservoir thickness at tectonic slope or syncline parts are the favorable exploration areas.

Characterization of reservoir properties and pore structure based on micro-resistivity imaging logging: Porosity spectrum, permeability spectrum, and equivalent capillary pressure curve

According to the capillary theory,an equivalent capillary model of micro-resistivity imaging logging was built.On this basis,the theoretical models of porosity spectrum(Ф_(i)),permeability spectrum(K_(i))and equivalent capillary pressure curve(pe)were established to reflect the reservoir heterogeneity.To promote the application of the theoretical models,the Archie's equation was introduced to establish a general model for quantitatively characterizing bi,K,and pei.Compared with the existing models,it is shown that:(1)the existing porosity spectrum model is the same as the general equation of gi;(2)the Ki model can display the permeability spectrum as compared with Purcell's permeability model;(3)the per model is constructed on a theoretical basis and avoids the limitations of existing models that are built only based on the component of porosity spectrum,as compared with the empirical model of capillary pressure curve.The application in the Permian Maokou Formation of Well TsX in the Central Sichuan paleo-uplift shows that the Ф_(i),K_(i),and p_(ci) models can be effectively applied to the identification of reservoir types,calculation of reservoir properties and pore structure parameters,and evaluation of reservoir heterogeneity.

贵州西部二叠系茅口组顶部古风化壳型铁矿地球化学特征及控制因素

贵州西部中、上二叠统界线附近普遍发育与玄武岩风化壳有关的铁-多金属矿化。本文通过对贵州西部普安地区二叠系茅口组顶部古风化壳型铁矿详细野外地质调查发现,玄武岩风化壳剖面普遍发育铁矿化,但铁矿体主要呈透镜体产于喀斯特洼地或溶斗之中,剖面上表现为Fe由上而下运移富集的特征。7件岩(矿)石样品地球化学分析表明,贵州西部普安地区二叠系茅口组顶部玄武岩风化壳普遍富集Fe、Ti、Al的氧化物,As、Sb、Cu、Nb、V、U、Zr等亲硫及高场强元素。该区部分样品TFe2O3、TiO2含量达到工业品位,Al2O3也呈不同程度富集,具有铁-多金属富集的特征。此外,研究区Fe倾向于喀斯特洼地富集,而Ti、Al则倾向于残留喀斯特斜坡富集,具Al、Ti与Fe差异性分离的特征,这可能与玄武岩间歇性喷溢,古地理特征,风化作用及频繁海侵、海退事件关系密切。综合研究表明,峨眉山地幔柱活动推动了贵州西部古地理格局的演化(喀斯特不整合洼地),并造就了(矿源层)高铁钛玄武岩堆积。贵州西部二叠系茅口组顶部古风化淋滤作用及古地理格局演化对Fe的迁移及富集具有显著的控制作用。因此,对于贵州西部二叠系茅口顶部玄武岩风化壳及其古地理恢复的研究是寻找玄武岩风化壳型铁矿床的重要基础。

Characteristics of “Guangyuan-Wangcang” trough during Late Middle Permian and its petroleum geological significance in northern Sichuan Basin, SW China

Based on outcrop characteristics, combined with regional tectonic background, drilling and geophysical data, the sedimentary characteristics of the Middle Permian Maokou Formation in the northern Sichuan Basin was studied by means of macroscopic and microscopic observation, geochemical element test, total organic carbon content and vitrinite reflectance measurement. There is a set of deep-water sediments rich in organic matter in the Guangyuan-Wangcang area of northern Sichuan during the late depositional period of the Middle Permian Maokou Formation. The strata are distributed from northwest to southeast, with thickness of 10–30 m, mainly composed of siliceous rocks and siliceous mudstones, intercalated with gravity flow deposits. Siliceous rocks and siliceous mudstones are characterized by thin single layer, flat bedding and rich siliceous radiolarians, calthrop and brachiopod with small body and thin shell, belonging to the typical sedimentary characteristics of deep-water trough facies. The contents of Cu, Co, Mo, Ni and the ratio of Ni to Co in the geochemical tests all indicate that the siliceous rocks are products of deep-water reducing environment. The TOC value ranges from 3.21% to 8.19%, with an average of 5.53%, indicating that the siliceous rocks have good hydrocarbon generation ability. The south side of the trough is in platform margin facies with high energy, and the sediments are mainly thick massive micritic-calcsparite biogenic(clastic) limestone, which is conducive to the formation and evolution of the reservoir. During the late sedimentary period of the Maokou Formation, the northward subduction and extension of the oceanic crust at the northwestern margin of the Yangtze Plate provided the internal dynamic conditions for the formation of the "Guangyuan-Wangcang" trough. According to the location, sedimentary characteristics and formation dynamics of the trough, it is similar to the "Kaijiang-Liangping" trough during Late Permian proposed by previous researchers. It is believed that the "Kaijiang-Liangping" trough already had its embryonic form during the Late Middle Permian.

Distribution and gas-bearing properties of Permian igneous rocks in Sichuan Basin, SW China

Based on the analysis of outcrop, seismic, logging and drilling data, combined with exploration practice, the characteristics,distribution, reservoir performance and gas-bearing properties of Permian igneous rocks in Sichuan Basin are studied. The study shows that central volcanic eruptive facies are developed in Sichuan Basin, and their lithological assemblages and distribution characteristics show obvious differences. The igneous rocks are mainly distributed in three regions: the southwestern part of the basin has dominantly largescale overflow facies basalts; the central and western part of the basin, Jianyang-Santai area, develop intrusive rocks, volcanic lavas(basalts)and pyroclastic rocks; and the eastern part of Sichuan, Dazhou-Liangping area, only develop diabase and basalts. Five aspects of understandings are achieved:(1) The Upper Permian igneous rocks can be divided into intrusive rocks and extrusive rocks, with the extrusive rocks as the main body. The chemical compositions of the extrusive rocks are characterized by both alkaline basalt and tholeiitic basalt, and belong to the subalkaline type of transitional basalt magma eruption.(2) There are obvious rhythmic structures vertically among overflow facies basalt, and the single rhythmic layer consists of, from bottom up, pyroclastic rocks(undeveloped), gray and dark gray porphyritic basalts(unstable), dark gray and purple microcrystalline-cryptocrystalline basalts, dark greyish green porous and amygdaloid basalts; the central volcanic eruption shows the rhythm and the vertical sequence of volcanic clastic rocks(agglomerates and breccias), volcanic lava, tuffaceous lava from bottom to top.(3) The pore types of basalt and pyroclastic rocks are diverse, mainly dissolution pore and de-vitrification micropore, but their physical properties are different. Basalt is characterized by ultra-low pore permeability, small reservoir thickness, and reservoirs are distributed in the upper and middle parts of the cycle, with poor lateral comparability. Volcanic clastic rocks are medium to high porous reservoirs(Well YT1: porosity: 8.66%?16.48%, average 13.76%) with large thickness and good reservoir quality.(4) Natural gas in basalts in southwestern basin mainly comes from Middle Permian, and natural gas in volcanic clastic rocks in central and western basin comes from Cambrian Qiongzhusi Formation.(5) Analysis of igneous reservoir-forming conditions in different areas shows that there are relatively insufficient gas sources and great differences in preservation conditions in southwestern basin.Reservoirs are poorly developed and gas-bearing is complex. The Jianyang-Santai area in the central and western part of Sichuan Basin has abundant hydrocarbon sources, developed reservoir, favorable preservation conditions and favorable gas geological conditions, and it is a favorable area for gas exploration.

Sedimentary Features and Paleogeographic Evolution of the Middle Permian Trough Basin in Zunyi,Guizhou,South China

Based on detailed field surveys,numerous sections and drillcores as well as analyses of lithofacies and sedimentary facies,sedimentary differentiation in the Middle Permian Zunyi(northern Guizhou,South China)trough basin was studied in detail.Caused by activities of the Emeishan mantle plume,the regional extensional setting and the extinguishing of carbonate production factories were shown from Guizhou to Yunnan.Three types of sedimentary enironments were developed in the study area:platform,slope,and basin.In this work,we established a sedimentary model for the study area,identified 14 syndepositional faults,and created the detailed reconstruction of the trough basin via a sedimentological investigation.It is found that the paleogeographic evolution of the study area can be divided into six stages:(1)a pre-rifting stage,(2)rapid rifting and subsidence,(3)stable rifting and subsidence,(4)the second stage of rifting and subsidence,(5)silicification,and(6)regional uplift and denudation.The sedimentary features and paleogeographic evolution of the Maokou Formation in Zunyi show that the platform,slope,and basin sedimentary features coexisted.In addition,the extensional setting and extinguishing of carbonate production factories were important preconditions for the evolution of the platform region in the study area,and the activity of syndepositional faults also played an important role in the paleogeographic evolution of this area.

Petrology and geochemical characteristics of dolomite in the Middle Permian Maoukou Formation,central Sichuan

Dolomites of the Maokou Formation in the central Sichuan were mainly developed in the middle-upper part of Member 2 and the lower part of Member 3 of Maokou Formation,it could be divided into fine-to medium-grained calcareous dolomite,fine-to medium-grained dolomite and breccia dolomite according to petrology characteristic,and the latter two were dominant.Through contrast of geochemical characteristic between micritic limestone and different types of dolomite,the dolomite was characterized by low Fe content,high Mn content and low Sr content relative to the micritc limestone.The micritic limestone and different types of dolomite had similar rare earth element(REE)distribution patterns which were characterized by depletion of light REE,weak positive anomaly of La and negative anomaly of Ce,and the dolomite also had weak positive anomaly of Eu.Compared with the micritic limestone,the fine-to medium-grained dolomite and breccia dolomite had low values of δ^(18)O and obvious high ratios of^(87)Sr/^(86)Sr.REE distribution patterns of different types of dolomite indicated that some geochemical characteristics of primitive limestone were preserved during the dolomitization process in the Maokou Formation,while weak Eu positive anomaly of dolomite and isotopic difference in dolomite and limestone might be caused by high-temperature thermal fluids during the dolomitization process.The dolomitization of the Maokou Formation was controlled by some factors:(1)strata residual seawater and hydrothermal fluids derived from magmatism were major sources of Mg during the dolomitization process;(2)heating effect of Emeishan large igneous province provided abnormal high paleogeotemperature for dolomitization;(3)fault systems offered rapid migration channels for abnormal geothermal convection and dolomitization fluid;(4)limestone of grain-shoal facies with good porosity and permeability was more prone to dolomitization than micritic limestone.Abnormal geotemperature and abundant Mg supply derived from Emeishan Large Igneous Province were key factors for dolomitization in the Maokou Formation,and different types of dolomites were formed in different periods of the dolomitization process.