J. Cent. South Univ. (2020) 27: 3375-3387

DOI: https://doi.org/10.1007/s11771-020-4553-1

Feature, genetic model and distribution of calcareous insulating layers in marine strata of western Pearl River Mouth basin in north of South China Sea

HU Yu(胡余)^{1, 2, 3}, ZHANG Ting-shan(张廷山)^{1, 2, 3},LIAO Ming-guang(廖明光)^{1, 2}, ZHU Hai-hua(祝海华)^{2, 3}

1. Natural Gas Geology Key Laboratory of Sichuan Province, Southwest Petroleum University,Chengdu 610500, China;

2. School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China;

3. Sichuan Key Laboratory of Shale Gas Evaluation and Exploitation, Chengdu 610091, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: We have systematically investigated the feature, genetic model and distribution of calcareous insulating layers in marine strata of the I oil group in member 2 of Zhujiang formation (ZJ₂I oil formation), western Pearl River Mouth basin (PRMB) in the north of the South China Sea by using data such as cores, thin sections, X-ray diffraction of whole-rock, and calcite cement carbon and oxygen isotopes. The lithology of the calcareous insulating layers in the study area is mainly composed of the terrigenous clastic bioclastic limestone and a small amount of fine-grained calcareous sandstone. On this basis, two genetic models of calcareous insulating layers are established, including the evaporation seawater genetic model and shallow burial meteoric water genetic model. The calcareous insulating layers of the evaporation seawater genetic model developed in the foreshore subfacies, mainly at the top of the 1-1 strata and 1-3 strata. The calcareous insulating layers of the shallow burial meteoric water genetic model developed in the backshore subfacies, primarily in the 1-2 strata.

Key words: western Pearl River Mouth basin; marine strata; calcareous insulating layer; genetic model

Cite this article as: HU Yu, ZHANG Ting-shan, LIAO Ming-guang, ZHU Hai-hua. Feature, genetic model and distribution of calcareous insulating layers in marine strata of western Pearl River Mouth basin in north of South China Sea [J]. Journal of Central South University, 2020, 27(11): 3375-3387. DOI: https://doi.org/10.1007/s11771-020- 4553-1.

1 Introduction

Intercalation is an impermeable layer that regulates the internal flow of fluid and divides the reservoir into multiple semi-connected or disconnected flow units. Since the early 1990 s, researchers around the world have studied the concept of intercalation in reservoirs, and achieved a series of results. Currently, most Chinese scholars accept the idea of intercalation proposed by ZHANG et al in 2003 [1]. Intercalation can be divided into two concepts, i.e., the insulating layer and the interlayer. Insulating layers are impermeable layers in reservoirs that prevent or control fluid movements. Their development is relatively stable, their range of distribution is usually more than half of the flow unit area, and their thickness varies greatly (tens of centimeters to a few meters as a minimum, tens of meters as a maximum). In the flow unit, the physical property boundaries of the insulating layers vary in different regions, which play a role in preventing the overwater and underwater channeling of oil and gas [2-4]. Interlayers are relatively impermeable or low permeable layers that have an obvious influence on fluid flow inside the reservoir in the process of oilfield development. Their development is unstable, while the range of distribution is usually less than half of the flow unit area, and their thickness is relatively small (typically just a few centimeters to tens of centimeters). The physical property of the interlayers is quite different from that of the main body of the reservoir, and the fluid movement cannot be controlled or prevented completely. According to the lithological and physical characteristics, the intercalation is divided into three types: calcareous intercalation, argillaceous intercalation, and physical intercalation [1]. The calcareous intercalation lithology consists of calcirudite, calcareous sandstone, calcareous siltstone, calcilutite, and calcium limestone. The argillaceous intercalation lithology comprises mudstone, silty mudstone, and argillaceous siltstone. Physical intercalation lithology is a conglomerate or a sandstone supported by the hetero basement and a medium conglomerate supported by the particles.

At present, most oil fields in China have entered the middle and late stages of exploration and development, with a high reservoir moisture content and a rapid decline in production. The key to the stable production of oilfields is to predict the residual oil distribution. The intercalation distribution is one of the main controlling factors affecting the residual oil distribution. The intercalation enhances the heterogeneity of the reservoir and makes the oil-water movement complex and variable, resulting in the sporadic distribution of the residual oil and difficulty in production [5]. Chinese scholars mostly use the methods of logging identification and interwell modeling methods to study intercalation in the strata of continental and transcontinental facies (especially the fluvial and delta facies) [6-8]. However, the study on the genetic model and distribution of the intercalation in marine sandstone reservoirs is relatively rare.

The western PRMB, situated in the northern part of the South China Sea, has a sedimentary deposition of the ZJ₂I oil formation, which is a typical marine non-barrier wave-controlled sandy littoral deposit. In this sedimentary body, there are multiple stages and complex causes of the calcareous insulating layer, the argillaceous insulating layer, and the physical interlayer. The calcareous insulating layers have a significant effect on the residual oil distribution at the top of this reservoir. In this research, data such as cores, thin sections, X-ray diffraction of whole-rock, and calcite cement carbon and oxygen isotopes were used to systematically study the feature, genetic model, and distribution of calcareous insulating layers in this stratum. The present studies have a significance impact on the detailed characterization of the reservoir, the distribution of residual oil, and the stimulation and potential exploration measures in the middle and late stages of the development oilfields.

2 Regional geological background

The Pearl River Mouth basin (PRMB) is an extensional basin with Cenozoic sediments in the northern continental margin of the South China Sea. The structure of the western PRMB mainly consists of two secondary tectonic units, the Zhu-3 depression and Shenhu uplift. The Zhu-3 depression can be divided into nine tertiary tectonic groups [9], which are Yangjiang B sag, Yangjiang horst, Yangjiang A sag, Yangjiang low bulge, Qionghai sag, Qionghai low bulge, Wenchang A sag, Wenchang B sag, and Wenchang C sag.

The study area is located in the eastern Wenchang B sag of the Zhu-3 depression. The main oil reservoir is the ZJ₂I oil formation of the Neogene lower Miocene. Before the Zhujiang formation period, basin basement sank significantly; the sea level continued to rise, the tectonic activity and fault-block movement were relatively calm. In the early stages of the Zhujiang formation period, the South China Sea movement in the PRMB led to frequent faulting activities in the Zhu-3 depression and to the formation of multiple fault depressions, which increased the capacity for formation. Then the basin basement continued to sink, and the sea level continued to rise. However, with the increase in sediment influx, the deposition rate was higher than the rate of the growth of the accommodation for the formation, and the base level continued to decline. As a result, the scattered fault depression center in the Zhu-3 depression has become a unified sedimentary basin, and it showed an asymmetric semi-cycle of base-level, which declines vertically. Changes in sea level, provenance supply, and tectonic movement led to the development of marine clastic littoral facies sedimentary system in Zhujiang formation of Wenchang B sag [10, 11], which mainly developed four subfacies, including shoreface, foreshore, backshore and coastal dune [12].

The lithology of the ZJ₂I oil formation is mainly terrigenous clastic bioclastic limestone, fine-grained calcareous sandstone, and gray mudstone. The sequence stratigraphy of the ZJ₂I oil formation includes only one parasequence set, three parasequences, and eight strata groups (Figure 1). There are three kinds of intercalations: calcareous insulating layers, argillaceous insulating layers and physical interlayers. According to logging interpretation results, the oil-bearing formation is mainly 1-1, 1-2, and 1-3 strata groups at the top of the ZJ₂I oil formation, and the internal intercalations is mostly calcareous insulating layers.

Figure 1 Composite histogram of well 1

3 Methods and materials

This research uses a systematic approach that integrates the concepts of mineralogy, petrology, elemental geochemistry, sedimentology, and sequence stratigraphy. This study includes observations of the core in four wells (well 1, well 2, well 6 and well A6, with a total core length of 131.56 m/(431.63 ft)), and thin sections (33 samples). It also includes a study of the feature of calcareous insulating layers by X-ray diffraction of whole-rock (20 samples) in which the source materials of carbonate cement are studied with calcite cement carbon and oxygen isotopes (15 samples). The identification of thin sections and the analysis of X-ray diffraction of whole-rock, both were performed at the experiment and Analysis Center of school of Geoscience and Technology at the Southwest Petroleum University, China. The researches on calcite cement carbon and oxygen isotopes took place at the State Key Laboratory of Geo processes and Mineral Resources of the China University of Geosciences (Wuhan).

4 Results and discussions

4.1 Feature of calcareous insulating layers

4.1.1 Lithology

In the present study, calcareous insulating layers in the ZJ₂I oil formation are mainly formed by carbonate cementation. Based on core observations and analysis of the whole-rock mineral composition (Table 1), it is understood that the lithology of calcareous insulating layers mainly consists of terrigenous clastic bioclastic limestone (Figures 2(a) and (b)) and a small amount of fine-grained calcareous sandstone. The bioclastic limestone and calcareous sandstone are composed of particles (bioclastic particles and terrigenous clastic particles) and interstitial materials (matrix and cement). The sedimentary structures are mainly massive structures, ripple marks, biological disturbance structures, etc. The internal stratification is not obvious, and the cementation is dense. Bioclastic particles are mostly foraminifera and gastropod (Figure 2(c)), while a small amounts of corals, brachiopods, flap gills, echinoderms (Figure 2(d)) and ostracods also exist. The terrigenous clastic particles are quartz, with few feldspars and rock debris (Figure 2(e)). The cement in the interstitial materials is mostly calcite (Figure 2(f)), followed by dolomite, with few siderites (Table 1).

Table 1 Statistical table of X-ray diffraction of whole-rock of calcareous insulating layers

Figure 2 Macroscopic and microscopic characteristics of calcareous insulating layers:

4.1.2 Rock structural characteristics

Generally, the structural characteristics of clastic sandstone can be described by the compositional and textural maturities. Compositional maturity, also known as chemical maturity and mineral maturity, refers to the degree of clastic components in sandstone that reach the most stable end product (quartz) under the influence of weathering, transportation, and deposition. Composition maturity is the reflection of geological conditions, weathering degree, and transportation distance in the source area. Generally, it is characterized by the quartz content of sandstone. Textural maturity, also known as physical maturity, refers to the degree to which clastic sediments approach ultimate structural characteristics under modification by weathering, transport, and deposition. The structural maturity increases with the increase in the number and distance of rehandling, and it is characterized by the separability, roundness, and clay (heterozygous) content of the clastic rocks. The structural characteristics of carbonate rocks are divided into four types of structures, i.e., granule form, biological skeleton, re-crystallized form, and secondary framework. The lithology of the calcareous insulating layers is primarily a carbonated rock (bioclastic limestone) with a small amount of quartz sandstone. The structural characteristics of calcareous insulating layers cannot be described solely from sandstone maturity or carbonate rock structure. In this article, the density, separation, contact relationship, and type of cementation were used to describe features of the rock structure characteristics of the calcareous insulating layers (Figure 3).

Through the observation of thin sections (33 samples) of the calcareous insulating layers, the following conclusions are drawn. The diagenesis process causes strong cementation and weak compaction, resulting in the presence of a lot of carbonate cementation, so that the calcareous insulating layers are generally dense. The separations are usually above average, and the overall separations are better. The contact relationship between most of the particles is point and line contacts, and a small portion is suspension contact, but concave-convex and suture contacts are missing. Pore cementation is the most important cementation, followed by base cementation, with the least contact cementation.

4.2 Genetic models of calcareous insulating layers

4.2.1 Carbon and oxygen isotope numerical analysis

The calcium source of the calcareous insulating layers determines its genetic model. This study analyzed the CO₃^2- source through the examination of carbon source, and then obtained its calcium source. In the burial diagenetic environment, many factors affect δ¹³C value, among which the most important is the source of pore fluid related to salinity [13-19]. The source of pore fluid is mainly meteoric water or seawater preserved by sediments during sedimentation. Therefore, to study the genetic model of the calcareous insulating layers is to find out the source of pore fluid.

Figure 3 Statistics of structural characteristics of calcareous insulating layers

The analysis of carbon and oxygen isotopes of calcite cement in the calcareous insulating layers (15 samples) from 4 coring wells at different depths in the study area (Table 2) showed that δ¹³C values of all samples ranged from -3.44‰ to 2.28‰, and δ¹⁸O values ranged from -8.77‰ to -4.31‰. The sediments in the study area are littoral facies, and the influence of fluvial action is weak, while that of tidal action is strong. Therefore, most of the calcium comes from the littoral tidal seawater. The primary sources of calcium are calcareous bioclasts, feldspathic quartzitic debris, and orthoclase. According to mineral statistics, the content of feldspathic quartzitic debris and orthoclase is small (Table 1), so the primary source of calcium is calcareous bioclasts.

The δ¹³C value of CO₂ dissolved in meteoric water is about -7‰, while the δ¹³C value of CO₂ dissolved in seawater is from -2‰ to 6‰, and pore fluids can come from both seawater and meteoric water [20-24]. The δ¹³C values of the pore fluid in the calcareous insulating layers range from -3.44‰ to 2.28‰ (Figure 4(a)). Besides, the δ¹³C values of only two samples are less than -2‰ (-3.44‰ and -2.16‰), while the δ¹³C values of the other samples are all more than -2‰, indicating that when the calcareous insulating layers were formed, the pore fluid was mainly seawater and a small amount of pore fluid was meteoric water.

The calcium source is controlled by aqueous media. In order to analyze the sources of the pore fluid forming the carbonate cement, the non-dimensional Z-value is calculated in this paper by the method of dividing marine facies and fresh water facies carbonate rock that was proposed by KEITH et al [25]. It is found that the paleosalinity of the aqueous media increases with the increase of δ¹³C and δ¹⁸O values. Furthermore, the results suggested that the pore fluid could be distinguished from seawater or meteoric water. The formula is as follows:

The properties of fluids in pores can be judged by the non-dimensional Z-value as follows: when Z>120, the fluids in the pores are obtained from seawater; when Z<120, the fluids in the pores are acquired from meteoric water; and when Z is close to 120, the fluids are obtained from both seawater and meteoric water [25]. Calcite cement carbon and oxygen isotopes (15 samples) of the calcareous insulating layers were used to calculate the Z-values (Table 2). The Z-values of 66.7% of the samples are more than 120, with an average value of 122.8, indicating that the pore fluid that forms carbonate cement is seawater. For only two samples, Z-values are less than 120 (117.6 and 119.2), showing that these two calcareous insulating layers intermittently exposed to sea level and that the pore fluid is meteoric water (Figure 4(b)).

Table 2 Calcite cement carbon and oxygen isotopes in the calcareous insulating layers

Figure 4 Distribution diagram of calcite cement carbon and oxygen isotopes and Z-values of calcite cements in calcareous insulating layers

4.2.2 Genetic models of calcareous insulating layers

Through the numerical analysis of calcite cement carbon and oxygen isotopes, it is found that the calcite cement carbon and oxygen isotopes in the calcareous insulating layer are distributed in two regions (Figure 4). The genetic models of the calcareous insulating layers in these two regions are defined as the evaporation seawater genetic model (region I) and shallow burial meteoric water genetic model (region II). The δ¹³C values of the two genetic modes are quite different, and the δ¹³C values of region II are significantly less than the δ¹³C values of the region I (Table 3). The calcareous insulating layers of the two genetic models are indistinct from each other by logging. They can only be separated by observation of thin sections and analysis of calcite cement carbon and oxygen isotopes.

1) Evaporation seawater genetic model

The calcareous insulating layers of this genetic model are mostly affected by seawater evaporation. Cementation is mainly middle stage ferrocalcite, and a small amount of middle stage ankerite or anhydrite. These types of cement are mostly formed in terrigenous clastic bioclastic limestone and filled in the remaining intergranular pores (Figures 5(a) and (b)). They are the products of metasomatism of quartz particles, and the debris particles are in line contact.

Table 3 Content of calcite cement carbon and oxygen isotopes of calcite cement in different genetic modes of calcareous insulating layers

In the foreshore subfacies, higher temperatures and intense evaporation lead to a gradual increase in the concentration of CO₃^2- in surface pore water. Under the influence of evaporation and capillary pressure, the pore water may appear an upward movement and continuously replenish CO₃^2- for the surface water. In the pore water near the surface, CO₃^2- oversaturation occurs, resulting in the diagenesis of sediments to form cement, which creates a calcareous insulating layer after burial. Besides, if the sediment is raised to the adjacent or exposed surface area in the later period, the calcareous insulating layer formed in the earlier period may be leached by meteoric water, forming dissolution pores. Under the intense surface evaporation, the newly formed dissolution pores may be re-filled with calcareous cement, forming the calcareous insulating layer of the evaporation seawater genetic model.

Figure 5 Microscopic characteristics of calcareous insulating layers:

2) Shallow burial meteoric water genetic model

The calcareous insulating layers of this genetic model are mainly developed in the intermittently exposed areas and formed in the freshwater environment during the shallow burial period. The primary cement is early stage calcite. It is usually precipitated directly from supersaturated alkaline water at standard pressure and temperature. Most of them are formed in terrigenous clastic bioclastic limestone with basement cementation and matrix support structure. The cementation is filled between the bioclastic particles (such as foraminifera and brachiopod) (Figures 5(c) and (d)).

With the development of the sedimentary process, the calcareous debris deposit in the preceding period entered the shallow burial stage in the backshore subfacies. They dissolve under the action of meteoric water leaching. A large amount of CO₃^2- reaches the pore water, resulting in CO₃^2- supersaturation of the pore water. Subsequently, the pore water filled with CO₃^2- flows along with the meteoric water in the reservoir, dissolving calcium in this process to maintain the CO₃^2- concentration. Under the condition of certain pressure and temperature, calcium precipitates form cement, and finally become calcareous insulating layers. Besides, calcareous debris dissolves in the shallow-buried sand body, and the concentration of CO₃^2- in local pore water increases gradually. After CO₃^2- supersaturation, calcareous concretions preferentially form in the biological body cavity or near the tubercle material (fine-grained debris). Subsequently, calcium cementation occurs constantly around calcareous concretes, causing them to grow continuously, and eventually develop into calcareous masses. If there is sufficient CO₃^2- in pore water, the calcareous masses gradually grow, and subsequently, the adjacent masses merge to form a fully consolidated calcareous insulating layer. If there is a lack of CO₃^2- in pore water (the growth of calcareous masses consumes a large amount of CO₃^2-, and there is no obvious bedding flow of pore water to supplement CO₃^2-), the concentration of CO₃^2- among the masses is low, and it is not enough to form a new mass. Finally, no calcium cementation occurs in the sandstone, and a transitional interface is formed. If there is a lack of CO₃^2- in pore water, there would not be any calcium cementation, and lithology is only sandstone.

4.3 Distribution of calcareous insulating layers

4.3.1 Relationship between distribution of calcareous insulating layers and sedimentary subfacies

The composition of sediment is controlled by the sedimentary environment [26-28]. The distribution map of sedimentary subfacies of the ZJ₂I-1 sand group [29] and the accumulated thickness contour map of calcareous insulating layers were superimposed. The results show that the calcareous insulating layers are more developed in the foreshore, less in the backshore, and almost not in the shoreface and coastal dune (Figure 6). The sedimentary environment of the ZJ₂I oil formation is littoral facies. From the sea to the land, there are four sedimentary subfacies: shoreface, foreshore,backshore and coastal dune. The enrichment of organisms decreases gradually. Biodetritus is the primary calcium source in the study area, and the concentration of biodetritus decreases gradually from shoreface to the coastal dune.

Figure 6 Composite diagram of sedimentary subfacies distribution and accumulated thickness of calcareous insulating layers in ZJ₂I-1 sand group

The calcareous insulating layers are dominated by the evaporation seawater genetic model in the foreshore and by shallow burial meteoric water genetic model in the backshore. In the foreshore, strong evaporation makes the sedimentary relatively rich in calcium, which can be used as a good source of calcium in the later stages. Compared with other subfacies, the calcareous insulating layers are most developed in the foreshore. There are many bioclasts in the backshore, and carbonate debris is used as calcium sources for dissolution and precipitation, resulting in the development of calcareous insulating layers. Due to the double influence of calcium source and genetic model, there are relatively few calcareous insulating layers in the shoreface and coastal dune.

4.3.2 Relationship between distribution of calcareous insulating layers and sequence stratigraphy

The sequence stratigraphy of the ZJ₂I oil formation is a parasequence set, belonging to the regressive system domain. The cyclical stratigraphy includes three parasequences and eight strata groups [11]. There are multiple sea surfaces in this parasequence set. When flooding occurs, the increase rate of accommodation is much higher than the increase rate of sediment influx. The low hydrodynamics and abundant oxygen of the seabed often lead to the enrichment of organisms, providing a good source of calcium for the subsequent formation of calcareous insulating layers. At this time, a widely distributed argillaceous insulating layers are formed. The ZJ₂I oil formation can be divided into three parasequences at the interface of the argillaceous insulating layer (Figure 7). Its top oil-bearing interval (ZJ₂I-1 sand group) has a low sea level, and part of the deposit is intermittently above the sea level. Under the control of calcium source, calcium preferentially accumulates near the sea surface, forming calcareous insulating layers. The ZJ₂I-1 sand group can be divided into four strata groups by using both calcareous and argillaceous insulating layers at the interface of the strata group [20].

The datum planes of the 1-1 strata group and the 1-3 strata group decrease, the water becomes shallower, and the hydrodynamics are enhanced. Waves have enough energy to transport large amounts of bioclasts to shallow water deposition, where evaporation and cementation of carbonate occur to form the calcareous insulating layers of the evaporation seawater genetic model. At the same time, calcium-laden water tends to flow along with layers, making such calcareous insulating layers continuously distribute. In the 1-2 strata group, the rise of datum planes makes the sediments intermittently expose above sea level, forming the calcareous insulating layers of shallow burial meteoric water genetic model. Besides, the strength of hydrodynamics in different regions is different, due to the variations in palaeogeomorphology, resulting in the unusual ability to transport sediments, which makes the calcareous insulating layers formed in this sedimentary have poor continuity and unstable distribution.

Figure 7 Cross-well comparison profile of intercalations

5 Conclusions

1) The lithology of calcareous insulating layers is dominated by terrigenous clastic bioclastic limestone, supplemented by a little calcareous sandstone. The rock consists of 24.30%-87.22% calcite, 6.24%-58.20% quartz, and less than 11.86% feldspar, along with richbioclast, which is dominated by foraminifera and gastropod. The diagenesis process causes strong cementation and weak compaction, resulting in the presence of a lot of carbonate cementation. Features of the rock structure characteristics are dense, good separation, point or line contact relationships, and pore cementation.

2) The calcium source is controlled by aqueous media. By analyzing the carbon and oxygen isotopes of calcite cement in the aqueous media, the genetic models of the calcareous insulating layers are defined as the evaporation seawater genetic model and shallow burial meteoric water genetic model. The primary influence factors of the δ¹³C values in the evaporation seawater genetic model are as follows: the concentration of CO₃^2- in surface pore water increases gradually due to the strong evaporation in seawater environment; the carbon isotope molecules with low δ¹³C value are easy to be missing; the carbon isotope molecules with high δ¹³C value move upward with the pore water under the capillary pressure to supplement the surface water with CO₃^2-, resulting in forming calcareous insulating layers with higher δ¹³C value. The primary influence factors of the δ¹³C values in the shallow burial meteoric water genetic model are as follows: the δ¹³C value is relatively low because of the enrichment of light carbon isotope in freshwater environment; under standard burial depth, pressure and temperature, CO₂ generated by the decomposition of organic matter in the upper and lower mudstone strata reduces the δ¹³C value; dissolution of light-carbon calcareous debris increases the concentration of CO₃^2- in local formation water, and CO₃^2- can continue to act as a carbon source to form calcareous concretion inside the body cavity or near the concretion, so that the change of the δ¹³C values is small.

3) From the plane distribution, the calcareous insulating layers are dominated by the evaporation seawater genetic model in the foreshore, and by shallow burial meteoric water genetic model in the backshore. From the vertical distribution, the former is primarily distributed in the 1-1 strata group and the 1-3 strata group, and the latter is mainly distributed in the 1-2 strata group.

Contributors

ZHANG Ting-shan and LIAO Ming-guang provided the concept. HU Yu conducted the literature review and wrote the first draft of the manuscript. ZHU Hai-hua provided the X-ray diffraction of whole-rock and calcite cement carbon and oxygen isotopes data. HU Yu analyzed the measured data, and finally completed the manuscript preparation and modification.

Conflict of interest

HU Yu, ZHANG Ting-shan, LIAO Ming- guang and ZHU Hai-hua declare that they have no conflict of interest.

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(Edited by YANG Hua)

中文导读

中国南海北部珠江口盆地西部海相地层钙质隔层特征、成因及分布

摘要：综合利用岩心、普通薄片、全岩和黏土矿物X射线衍射、碳氧同位素等资料，对珠江口盆地西部珠江组二段I油组海相砂岩地层钙质隔层的岩性和矿物结构特征、成因模式及其分布规律进行系统研究。研究区钙质隔层岩性主要为含陆源碎屑生屑灰岩和少量细粒钙质石英砂岩。在此基础上建立了两种钙质隔层的成因模式：蒸发作用海水成因和浅埋藏淡水成因。蒸发作用海水成因的钙质隔层在前滨亚相发育，主要发育在1-1岩层组和1-3岩层组顶部。浅埋藏淡水成因的钙质隔层在后滨亚相发育，主要发育在1-2岩层组。

关键词：珠江口盆地西部；海相地层；钙质隔层；成因模式

Foundation item: Project(51534006) supported by the Key Program of National Natural Science Foundation of China; Project(2014CB239005) supported by the National Key Basic Research and Development, China; Projects(41772150, 51674211) supported by the National Natural Science Foundation of China

Received date: 2020-01-18; Accepted date: 2020-07-03

Corresponding author: HU Yu, PhD Candidate; Tel: +86-18200143472; E-mail: 452404638@qq.com; ORCID: https://orcid.org/0000- 0003-4992-248X