(1. Jiangsu Oilfield Geological Science Research Institute, Yangzhou, Jiangsu 225012; 2. Postdoctoral Mobile Workstation in Jiangsu Oilfield, Yangzhou, Jiangsu 225012; 3. Jiangsu Oilfield Development Office, Yangzhou, Jiangsu 2250 12)
Author's brief introduction: Liu Jinhua, male, postdoctoral fellow of Jiangsu Oilfield Postdoctoral Mobile Station, engaged in petroleum and natural gas geology research. E-mail: joeco.com.cn Liu Jinhua
Abstract: There are different opinions on the origin of lacustrine carbonate rocks in the second member of Funing Formation on the west slope of Jinhu Depression. Therefore, this study focuses on the sedimentary facies of this layer, and through the study of the composition of micrite limestone, the preservation degree of worm tube, the distribution of stratum thickness and micrite limestone in the study area, it is proposed that a set of carbonate flat sedimentary system is developed in the study area, which is mainly composed of micrite limestone and bioclastic limestone in still water environment, including carbonate sheets. Planarly, the water body in the west of the study area is shallow, with strong hydrodynamic force, carbonate beach and dam facies, while the water body in the east is deep, with weak hydrodynamic force and carbonate sheet. On the basis of comprehensive research, the sedimentary model of carbonate plane sedimentary system is put forward
Key words: Jinhu sag; Rich 2 members; Carbonate rock; Sedimentary facies; Lacustrine facies
Study on sedimentary model of lacustrine carbonate rocks in the second member of Funing Formation on the western slope of Jinhu Depression
Liu Jinhua 1, 2, Tang Jiandong 3, Zhong Siying 1, Ge Zhengjun 1, Yao Fulai 1
(1. Jiangsu Oilfield Geological Research Institute of China Petrochemical Company, Yangzhou 225012; 2. Postdoctoral Mobile Station of China Petrochemical Jiangsu Oilfield, Yangzhou 225012; 3. Exploration and Development Department of China Petrochemical Jiangsu Oilfield Company, Yangzhou 2250 12)
Abstract: The origin of lacustrine carbonate rocks in the second member of Funing Formation on the west slope of Jinhu Depression is different, so the focus of this study is carbonate sedimentary facies. The main components of carbonate rocks are detritus and biological detritus, and other components are a small amount of oolitic and chromium particles. The particle content of rocks is different, and the content in different regions and strata is also different. The main component of rock is micrite matrix, and the cementing material of rock is ettringite cementing material. Through the analysis of carbonate rock composition, fossil preservation, formation thickness and distribution law of concealed rocks, it is found that fossils in this area are well preserved, the distribution law of formation thickness is obvious, and the formation thickness has little change; This aphanitic is distributed throughout the region. Carbonate platform is a sedimentary system composed of bioclastic limestone and subtle rocks, and it is a standard carbonate formation formed in strong and weak hydrodynamic environment, which can be divided into carbonate sheet, carbonate dam, carbonate beach and carbonate platform. On the plane, the water in the west of the work area is shallow and hydrodynamic, and carbonate dams and beaches are developed; In the east, the water depth is deep, the hydrodynamic force is weak, and carbonate terraces are developed. In the longitudinal direction, carbonate rocks are deposited from sub-layers, and the peak period of deposition is
Proceedings of the International Conference on Unconventional Oil and Gas Exploration and Development (Qingdao)
Sub-layer, and in the sub-layer, deposition formed a complete carbonate sedimentary system. Finally, the sedimentary model of lacustrine carbonate rocks is established. Key words: Jinhu sag; The second member of Funing Formation; Carbonate rock; Sedimentary facies; Lacustrine facies
introduce
The study of lacustrine carbonate reservoirs began in 1980s and has a history of more than 30 years. Previous studies on carbonate rocks such as Jurassic Daanzhai Formation in Sichuan Basin, Paleogene chunhua town Formation in Jiyang Depression in North China Basin, Paleogene Shahejie Formation in Huanghua Depression and Lower Cretaceous in Songliao Basin. Lacustrine carbonate rocks are mainly composed of reef facies and carbonate beach facies [1 ~ 4]. The lacustrine carbonate rocks in the second member of Funing Formation on the west slope of Jinhu Depression are a good set of oil and gas reservoirs. In the past, most of the researches on this composition were considered to be carbonate beach-bar facies deposits in shallow lakes. However, through the analysis of a large number of coring data, logging and mud logging data, it is found that there are great differences between carbonate rocks and carbonate beach bars, such as stable distribution in the bioclastic limestone, small thickness change and well-preserved biological shells, so the development model of carbonate beach bar sedimentary system is put forward. It provides a new geological basis for the next step of oilfield development and has important research significance for oil and gas exploration and development.
1 regional geological survey
Jinhu sag, located in the west of Dongtai sag in Subei basin, is a dustpan-shaped sag developed in the late Cretaceous. Jianhu Uplift in the north, Tianchang Uplift in the south, Zhangbaling Uplift in the west and Lingtangqiao-Liubao Low Uplift in the east, covering an area of about 5000km2 (Figure 1). The western slope of Jinhu sag refers to the western slope belt of Sanhe and Chajian sub-sag, with Jianhu uplift in the west and Zhangbaling uplift in the south. As a whole, the slope rises to the west and inclines to the northeast, geographically spanning Jiangsu and Anhui provinces, with a total area of about 1500km2 (Figure 1) [5 ~ 9].
Figure 1 Location Map of West Slope Work Area in Jinhu Depression, Subei Basin
The strata of Funing Formation in the study area are in pseudo-integration-integration contact with the underlying Taizhou Formation, and in pseudo-integration contact with the overlying Dai Nan Formation, which are called Fu 1 member, Fu 2 member, Fu 3 member and Fu 4 member respectively from bottom to top. Fu-2 member is the beginning of the maximum water invasion at the end of Fu-1 member, and three sub-members are formed from top to bottom: the first sub-member is dark mudstone member with a thickness of 60 ~ 120 m, which is a set of source rocks; The second sub-member is called "carbonate rock layer", which is the interbed between bioclastic limestone and calcareous mudstone, including dolomitic limestone, biolimestone and silty mudstone. It is also a set of source rocks with a thickness of 10 ~ 30m. The third section is a "mountain-shaped" section, which is composed of sandstone and mudstone mixed with a small amount of biological limestone, with a thickness of 60 ~ 80m, and is the main oil and gas reservoir in Jinhu Depression. The target interval of this study is the second sub-interval "carbonate rock layer".
Petrology and fabric characteristics of carbonate rocks
Lacustrine carbonate rocks are composed of carbonate particles, gypsum and other sediments, which are solidified into different rock types in a certain way through diagenesis. Petrology and fabric characteristics of carbonate rocks are important basis for distinguishing sedimentary environment of carbonate rocks. In this study, the particles, matrix and cement of rock are analyzed.
2. 1 particle characteristics
Based on conventional analysis methods and previous research results, the basic characteristics of clastic, bioclastic, oolitic and aggregate in carbonate rocks in the study area are analyzed.
(1) Internal debris: Internal debris mainly refers to gravel debris, sand debris and silt debris. Gravel debris is the product of re-crushing of diagenetic rocks (or semi-consolidated-quasi-consolidated sediments) [10], and its internal structure is bound to be restricted by diagenetic rocks and sediment types. Common rocks include micrite gravel limestone, gravel silty limestone and gravel wormhole limestone.
Gravels in the study area are of different sizes, with a particle size of 2 ~ 15 mm, changeable shapes, poor sorting and sub-edge-sub-circle roundness. Gravel detritus content is 7% ~ 65%, mainly composed of micrite calcite and dolomite, micrite structure, micrite structure contains particles, and the filler between gravels is micrite calcite, dolomite or oolite.
Sand chips are widely distributed in this area, with a content of 5% ~ 55%. The main component is micrite calcite, followed by dolomite, with irregular shape (plate-1), poor sorting and poor roundness, with a particle size of 0.1~ 0.2 mm. Most of the sand debris is filled with micrite calcite, with a small amount of micrite dolomite or terrigenous debris. Common rocks include micrite sand mixed with argillaceous limestone, gray flash sand algae mixed with dolomite limestone, sand micrite dolomite limestone and so on.
(2) Biodetritus: Biodetritus is mainly composed of worms, algae, foraminifera and algae piles, followed by ostracods, crustaceans and gastropods [1 1, 12]. The biological species are relatively single, but the content of biological debris is relatively high and varies greatly, with the content of 1% ~ 80%.
The content of wormlike biological debris in carbonate rocks is 30% ~ 70%. Most of the worm tubes are well preserved, distributed in clusters, and occasionally distributed in parallel bedding, which has certain directionality. The cochlear duct is filled with bright calcite and powder debris; The wall of the tube is mostly bonded with small balls or powder crumbs, which makes the surface of the tube shell tumor-like (plate-1); The tubes of some insects are covered by algae, which makes the tube wall form a thickened ring. The cochlear duct is 0.5 ~ 2.5 mm in diameter and has a biological skeleton structure. Related particles include gravel, sand and algae. Worm tube fragments are mostly filled with micrite calcite, and occasionally with bright calcite. Common rocks are micrite limestone, microcrystalline limestone, muscovite or muscovite.
Algae or algae chips are relatively developed in carbonate rocks, with a content of 3% ~ 85%, and have a layered concentric layer and a radiation band structure. Dark and transparent, composed of micrite calcite and dolomite, with a small amount of argillaceous. The shape is hemispherical, wavy or small monomer column.
(3) Oolitic particles: Oolitic particles are important particles in carbonate rocks in this area, with the content of 1% ~ 70%. The common oolitic grains are algae oolitic grains, normal oolitic grains, negative oolitic grains, repeated oolitic grains, crystalline oolitic grains and deformed oolitic grains.
(4) Aggregate: The aggregate content in rocks in this area varies greatly, ranging from 5% to 45%. Different shapes, ranging in size from 0.06 mm to 2 mm, with poor roundness and poor sorting, are composed of silty sand or micrite calcite and dolomite, with dark color, good roundness and good sorting.
2.2 Mud crystal matrix and bright crystal cement
Micky limestone is a common interstitial substance in carbonate rocks in this area, and its content can vary from 0 ~ 100%. It is mainly composed of micritic Shi Ying, followed by micritic dolomite, and it is locally recrystallized in blocks or strips.
Mud crystals and particles are deposited at the same time, and its existence or content is an index to measure the energy of water during deposition. Mortar (mud crystal) is the product of low-energy environment, and the accompanying particles are also low-energy particles.
Bright crystal cement is formed by chemical precipitation of water between grains after grain deposition, which is characterized by cleanliness and brightness and is the product of diagenetic period. Bright crystal cements in this area generally have the structure of the first and second generation cements, and the content of bright crystal cements is 0 ~ 40%, which is mainly composed of bright crystal calcite, followed by dolomite, and the matrix content is generally greater than that of bright crystal cements.
3 sedimentary characteristics and sedimentary model
Through core observation, analysis and test and single well equivalence study, the second member of the second member of Fu 'er Formation in the west slope of Jinhu Depression
Proceedings of the International Conference on Unconventional Oil and Gas Exploration and Development (Qingdao)
Carbonate rocks are mainly developed, which are mainly composed of a set of biological limestone deposited in brackish water environment, supplemented by micrite limestone. Carbonate rocks have the following characteristics: (1) Insect tubes in the stratum are well preserved, but they are not destroyed in most areas; (2) The distribution of stratum thickness is regular, with little change in thickness and no obvious dam characteristics; (3) Carbonate strata are widely distributed in the whole area, which is also inconsistent with beach bars; (4) micrite limestone is distributed all over the region, and its content is generally high, so it is difficult to deposit in strong hydrodynamic environment. In view of this, this study believes that the deposition of the target layer in this area is due to carbonate flat deposition; Moreover, according to the research of Yu Changmin and Wang Huiji (1976) of Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, the worm-like tube fossil in the target layer was identified as a living tube fossil of polychaete, a secluded subclass of annelids. Modern polychaetes all inhabit, mainly living at a depth of 60 meters from the coast to the water. Therefore, it can be said that the water environment of carbonate rock deposition in the second member of Fu in the work area should be deep. This study believes that there is a set of carbonate plane sedimentary system in the work area. Carbonate rock flat is a set of carbonate rock deposits developed in the slope zone of the basin, lakeside and shallow lake area, including four microfacies of carbonate rock sheet, carbonate rock dam, carbonate rock beach and mud flat (Figure 2), belonging to the sedimentary system dominated by micrite limestone and bioclastic limestone in still water environment.
Fig. 2 Phase model diagram of carbonate plane sedimentary system
Carbonate dam: This microfacies is developed at the edge of the work area and is obviously influenced by lake waves, with a thickness of 15 ~ 40m, mainly accompanied by micrite limestone and bioclastic limestone, and broken biological shells formed by wave action can be seen in many places (Figure 3, Panel -3 and Panel -4).
Carbonate beach: Thin-layer carbonate rock developed in the same position as carbonate dams, but with small thickness, distributed longitudinally between thick-layer carbonate dams, with stable distribution, mainly accompanied by micrite limestone and bioclastic limestone (plate -5, plate -6), and biological debris is often broken or hydrodynamically reformed.
Carbonate slices: distributed in areas with deep water bodies such as Gaoji-Cui Zhuang, mainly composed of biolimestone, with well-preserved biocrusts and layered distribution, generally associated with grayish-black mudstone or shale (Figure 3, plates -7 and -8). Although carbonate beach and carbonate rock slice are both thin carbonate rocks, their development environments are different in water depth, which leads to different preservation degrees of biological debris and mud color.
From the core observation, it can be seen that carbonate rocks are deposited after sandstone deposition, followed by semi-deep lacustrine gray-black mudstone and shale deposition, forming a set of typical strata in which the water body changes from shallow to deep and from strong hydrodynamic to weak hydrodynamic. The sedimentary environment of carbonate rocks is between sandstone and gray-black mudstone, and its sedimentary environment should be between the static water environment of semi-deep lake and the strong hydrodynamic environment of sandstone, belonging to static water-weak hydrodynamic deposition.
Fig. 3 Phase sequence of carbonate plane sedimentary system
4 Distribution and evolution of sedimentary facies
From the profile and plane analysis, the flat sedimentary system of carbonate rocks in the study area is not thick, but it is thick in Nanhu-Fan Zhuang area, with the maximum thickness of about 40m, which gradually becomes thinner from the west edge of the slope to the southeast (Figure 4), and the relative content of carbonate rocks in the stratum gradually decreases until there is no carbonate rock development area in the river depth 1 well area, which is mainly controlled by lake water depth, hydrodynamic environment and sedimentary facies belt. Three sets of carbonate deposits are mainly developed in the carbonate strata of Ersha Formation in the second member of Funing Formation in the west slope of Jinhu Depression, so Ersha Formation is divided into three sublayers from bottom to top (hereinafter referred to as 3, 2 and 1 sublayer). According to the specific situation of the second layer, the plane distribution of sedimentary facies in the work area is analyzed.
Phase diagram of east-west connecting well profile on west slope of Jinhu.
In these two sub-layers, carbonate beach bars are mainly developed in two areas, namely, Well Fan 7-South 1 Well Lu 1 Well Hong 2, Well Cui 17-Song 1 Well, and form two dams at a certain angle with the shoreline./Kloc-0. From Yang 3 well in the south of the study area to Cui 7 well in the middle, and then to Gao 6 and Gao 3 wells in the north, carbonate mat microfacies are developed in a large area. Mudflat microfacies only develop around Well Yangx7 in the south, east of Well Cui 4 and east of Well Heshen/Kloc-0 (Figure 5). From the plane analysis, the distribution of sedimentary facies is basically shallow water with strong hydrodynamic force in the west, with carbonate beach and dam facies, while deep water with weak hydrodynamic force in the east, with carbonate sheet, which is consistent with the understanding of single well and profile.
Fig. 5 Sedimentary facies plan of small layer on the west slope of Jinhu Depression
Vertically, the third layer is the stratum where carbonate rocks have just begun to deposit. Carbonate rocks are limited in distribution and only developed locally in Cui Zhuang and Gaoji. The thin thickness and the development of micrite limestone indicate that the salinity of water was relatively low at that time, which was not conducive to the large-scale deposition of carbonate rocks. The scale of 1 layer and 2-layer carbonate rocks is similar, and they are distributed in the whole region. In terms of thickness, layer 2 is larger than layer 1. Generally speaking, the carbonate rocks of Ersha Formation in the second member of Fu began to deposit in the third layer, reached the peak in the second layer, and 1 layer began to decline again, forming a complete lacustrine carbonate deposition system.
5 conclusion
(1) The carbonate rocks in the study area are mainly composed of internal debris and biological debris, with a small amount of oolitic grains and aggregates. The content of particulate matter is different, and there are great differences in different regions and different horizons. The matrix is micrite matrix, and the cements are mostly bright crystal cements.
(2) The wormpipes in the carbonate formation of the second member of Fu are well preserved, and the thickness of the formation is distributed regularly with little change, and micrite limestone is distributed in the whole area; In this study, it is proposed to develop a plane sedimentary system of carbonate rocks in the study area. Carbonate rock flat is a sedimentary system dominated by micrite limestone and bioclastic limestone in still water environment, including four microfacies: carbonate sheet, carbonate dam, carbonate beach and mud flat.
(3) On the plane of Ersha Formation in the second member of Fu, the water body in the west is shallow with strong hydrodynamic force, and carbonate beach and dam facies are developed, while the water body in the east is deep with weak hydrodynamic force and carbonate sheets are developed; In the vertical direction, carbonate strata began to deposit from small layers, reached the peak of deposition, and then began to decline, thus forming a complete lacustrine carbonate deposition system.
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Chart-1 Cui/Well Kloc-0/4 17 19.8 1m, bioclastic limestone, ×50.
Banerfan/Well KLOC-0/31409.34m oolitic limestone× 25
Fan Ban 3 13, bioclastic limestone (carbonate dam)
Fan Ban 4 13, bioclastic limestone (carbonate dam)
Fan 6 of well Ban 5, bioclastic limestone (carbonate beach)
Nan 7 Well Plate 6, bioclastic limestone (carbonate beach)
Cui 1 1 of Well Ban 7, bioclastic limestone (carbonate thin slice)
Bioclastic limestone of Well Gao 6-2 Plate 8 (carbonate thin slice)