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Pore Fluid Evolution Influenced by Volcanic Activities and Related Diagenetic Processes in a Rift Basin: Evidence from the Paleogene Medium-Deep Reservoirs of Huanghekou Sag, Bohai Bay Basin, China
Volcanic activities exert a significant influence on pore fluid property and related diagenetic processes that substantially controlled reservoirs quality. Analysis of Paleogene medium-deep sandstones on the Huanghekou Sag provides insight into relating the diagenetic processes to pore fluid property evolution influenced by volcanic activities. Three distinct types of pore fluids were identified on the basis of an integrated and systematic analysis including core and thin section observation, XRD, SEM, CL, and trace element. Alkaline aqueous medium environment occurred in E2s1+2 where volcanic activities have insignificant influence on pore fluids, evidenced by typical alkaline diagenetic events such as K-feldspar albitization, quartz dissolution, feldspar dissolution, and carbonate cementation. During the deposition of E3d3, influx of terrestrial freshwater and alteration of ferromagnesian-rich pore water result in the formation of mixing aqueous medium environment through volcanic eruption dormancy causing zeolite dissolution, clay mineral transformation, and K-feldspar albitization. Ferromagnesian-rich aqueous medium environment developed resulting from the intensive hydrolysis of the unstable ferromagnesian minerals formed due to intense volcanic activities during E3d1+2 and corresponding predominant diagenetic processes were characterized by the precipitation and dissolution of low-silica zeolites. Therefore, the differential properties of pore fluids caused various diagenetic processes controlling reservoir quality.
It has been documented that thousands of drilled wells show the tremendously exploratory potential of deep sedimentary formations, which has recently drawn great attention of petroleum geologists worldwide (e.g., [1–3]). The definition of the medium-deep or deep reservoirs differs from basin to basin conventionally depending on basin type and exploration degree. Bohai Bay Basin, one of the most petroliferous basins in China , and the medium-deep reservoirs are significant exploration interests commonly defined as those Paleogene intervals at burial depth more than 2500 m (e.g., [5–7]) but in the study area of Huanghekou Sag it is generally referred to those not deeper than 3500 m due to special geological settings. A clear understanding of Paleogene reservoirs development is therefore of great significance for optimizing petroleum exploration and increasing reserves.
Pore fluid evolution exerts a critical role in fluid-rock interactions (e.g., [9–12]). Sources of pore fluids are variable which mainly derived from connate water , meteoric water [14–16], dehydration of minerals (e.g., gypsum or clay mineral [17–19]), deep compaction brines , and hydrothermal water introduced by igneous activities . In most cases, however, the pore fluids are initially only one of them or mixture of them geochemically with different signatures. Thus, the pore fluids are easily altered by dissolution and precipitation of minerals, expulsion of organic CO2 and acids, volcanic eruption (e.g., [15, 16, 21, 22]), or mixing with other fluids (e.g., abundance of ferromagnesian caused by basalt eruption) during progressive burial. Different fluid sources and interactions in the reservoir pore have a vital impact on pore fluid property evolution, and thus, to a large extent, will influence the diagenesis and reservoir quality (denotes reservoir porosity and permeability in this paper). Therefore, pore fluid evolution and related diagenetic processes are complicated and variable [2, 23], which can induce strong reservoir heterogeneity [24, 25]. In an attempt to shed light on pore fluid evolution during progressive burial it is thus essential for predicting diagenetic processes and development of hydrocarbon reservoirs (e.g., [10, 26]).
A large number of case studies associated with pore fluid evolution and diagenesis have been made over the past decades, with most works focused on the small scale sandstone reservoirs with nonvolcanic development [9–12, 21, 22]. So far, however, there are few reports that have focused on the reservoir quality in the study area sandstone influenced by volcanic activities but had not investigated the pore fluid evolution associated with volcanic activities and their impacts on diagenetic process . The study area of Huanghekou Sag had undergone multiphase of volcanic eruption during the Paleogene where the aqueous medium is strongly altered by hydrolization of chemically unstable ferromagnesian minerals. Various types of pore fluids developed in Paleogene medium-deep reservoirs (E2s1+2, E3d3, E3d1+2), which controlled the reservoir quality that is better than that of adjacent areas with no volcanic eruption, providing an excellent example to investigate the properties of pore fluid aqueous medium evolution influenced by volcanic activities. The objectives of this study are to understand pore fluid evolution influenced by volcanic activities, relate the diagenetic processes to pore fluid property evolution, and evaluate controls of different diagenesis on reservoir quality. These objectives will be achieved by an integrated analysis of petrology, mineralogy and trace element compositions combined with a range of analytical techniques. The study provides a useful analogue for understanding the pore fluid evolution of the medium-deep reservoirs that share similar geological settings.
2. Geological Setting
The Bohai Bay Basin, one of the most petroliferous basins in China , is located on the eastern coast of China (Figure 1), covering an area of approximately 200,000 km2 (77,220 mi2). The Bohai Bay Basin is a complex Cenozoic lacustrine rift basin that formed in the Late Jurassic through the early Tertiary on the north China craton. Generally, the tectonic evolution of the basin is subdivided into two major stages ([28–30]; Figure 2), namely, a synrift stage between 65 and 24.6 Ma and a postrift stage from 24.6 Ma to the present, respectively. The Bohai Bay Basin contains several subbasins (Figure 1(a)). The Huanghekou Sag is located in the southern part of the Bozhong Subbasin of the Bohai Bay Basin and has an area of approximately 3,600 km2 (1,390 mi2), bordered in the north by the Bonan Uplift and in the south by the Laibei Uplift (Figures 1(a) and 1(b)).
Sediments filled in the Huanghekou Sag, in ascending order, are composed of the the Paleogene Kongdian (Ek), Shahejie (Es) and Dongying (Ed) formations, Neogene Guantao (Ng) and Minghuazhen (Nm) formations, and the Quaternary Pingyuan (Qp) Formation (Figure 2). The Paleogene Shahejie (Es) interval in the Huanghekou Sag is subdivided into four units (E2s4, E2s3, E2s2, and E2s1 [the first member of the Paleogene Shahejie Formation]), whereas the Dongying (Ed) Formations are composed of three units (E3d3, E3d2, and E3d1 [the first member of the Paleogene Dongying Formation]). Note that E2s2 and E2s1 intervals are referred to as E2s1+2 for short in this paper and E3d2 and E3d1 intervals are referred to as E3d1+2 because of similar deposition settings. The Paleogene Shahejie (Es) and Dongying (Ed) Formations, apart from the E2s4 and E2s3 interval, are the subject of this study. During the deposition of E2s1+2 intervals, braided river deltas developed overall the study area with volcanics distributed sporadically at the bottom and top of the E2s2 interval and drilled thickness approximately 26 m accounting for 13.16 percent of the total formation thickness of the E2s2 interval. However, there is no significant volcanics drilled for E3d3 that experienced rapid subsidence and lacustrine mudstones are developed in the central of the Huanghekou Sag interfingered with more proximal sandstones of braided river delta. With the continuous volcanic eruption of the lower unit of the second member of the Paleogene Dongying Formation , high paleogeomorphology developed around the well BZ-1 of the research area, which impeded the supply of clastic materials and development of braided river delta. Volcanic eruption in the E3d1+2 intervals is characterized by more active, multiperiod eruption and widespread distribution . According to the drilling results of well BZ-1, volcanics are drilled up to 91.7 m with percentage of 21.3 of the total formation thickness of the E3d1+2 interval. Examination of thin section in volcanics drilled in the study area indicates that lithology of igneous is mainly basalt.
Most samples in the Paleogene E2s1+2, E3d3, and E3d1+2 intervals are mainly lithic arkoses. Sandstones of E2s1+2 intervals are fine to medium-grained and moderately sorted sandstones of subangular to subrounded psephicity. Based on point-count data (Table 1), detrital quartz (30%–33%) and feldspar (37%–41%) are the predominant framework grain and detrital feldspar includes K-feldspar and plagioclase. Detrital K-feldspar is more abundant than plagioclase. Rock fragments (24%–31%) are less significant and include metamorphic and igneous grains, with acidic igneous rock most. Comparing to E2s1+2 intervals, samples in E3d3 and E3d1+2 have similar framework compositions and rock fragments constituents, but the percentages of each component are substantially different. The content of detrital feldspar in E3d3 ranges between 32% and 45%, detrital quartz varies from 25% to 35%, and rock fragments range from 25% to 39%. However, for E3d1+2, the contents of detrital feldspar, detrital quartz, and rock fragments are 33%–48%, 18%–35%, and 18%–46%, respectively.
|BZ-1 = Bozhong-1; Cmg = coarse-medium grained; E3d1 = first member of the Paleogene Dongying Formation; E3d3 = third member of the Paleogene Dongying Formation; E2s1 = first member of the Paleogene Shahejie Formation; E2s2 = second member of the Paleogene Shahejie Formation; Fg = fine grained; Fmg = fine-medium grained; Gs = grain size; Hg = heterogranular; Kf = K-feldspar; M = mica; Mcg = medium-coarse grained; Md = metamorphic detritus; Mfg = medium-fine grained; Mg = medium grained; Pl = plagioclase; P = psephicity; Q = quartz; Sa-Sr = subangular-subrounded; Sr = subrounded; Vd = volcanic detritus.|
3. Samples and Methods
The study was focused on the Paleogene medium-deep reservoirs (E2s1+2, E3d3, and E3d1+2) of Huanghekou sag where most of the producing oil fields occur and volcanic activities developed pervasively. Three main oil-producing wells drilled in the study area marked in black dotted line (Figure 1(b)). Previous studies demonstrated the fact that volcanic activities in the first and second members of Dongying Formation (E3d1+2) are characterized by more active, multiphase eruption and widespread distribution . Well- and seismic-based sedimentary facies and volcanics distribution of E3d1+2 indicate that the well BZ-1 located in volcanic overflow facies, surrounded by volcanic conduits displaying beads-shaped distribution (Figure 3). However, both well BZ-2 and well BZ-3 have no significant volcanic overflow facies developed. In other words, well BZ-1 may have experienced stronger influence of volcanic activities than that of the other two wells. Data from well BZ-1 are therefore of optimum choice for investigating the pore fluid evolution influenced by volcanic activities. Core and cuttings samples were collected from well BZ-1 borehole in the Paleogene at depths ranging from 2000 to 3500 m (6562–11,483 ft).
To quantify different diagenetic products, more than 60 polished thin sections and about 40 blue epoxy resin impregnated thin sections were prepared for rock mineralogy, diagenesis, and visual pore characteristics. Point counting was carried out on thin sections with at least 300 points to examine the correctness of the selected rock composition data. To determine the compositions of authigenic minerals and understand their spatial relationships, an amount of 15 representative samples was prepared using a Quanta 200 environmental scanning electron microscope (ESEM). Ten representative samples were viewed under a FEI Quanta 450 FEG field emission scanning electron microscope (BSE) equipped with backscatter detector (BSE) under an acceleration voltage of 20 kV and sample dip of 70°. In addition, different mineral phases were determined based on highly magnified backscatter electron image methods.
Reconstruction of paleosalinity based on boron in clays is an effective approach documented by using sediments at various time scale [31–34]. Boron-derived paleosalinity reconstructions are based on the direct positively linear relationship between boron in ancient clays and the paleosalinity of the depositional environment [34, 35]. Hence, to restore the paleosalinity, trace element analysis was carried out using ELE-MENTXR plasma mass spectrum analyzer with ELANDRC-e, measured following the rules of Methods for Chemical Analysis of Silicate Rocks-Part 30: Determination of 44 Elements (National Standard GB/T 14506.30-2010). Measured temperature and relative moisture for trace element are 20°C and 30%, respectively.
Samples were collected for mineralogical analyses using X-ray diffraction (XRD). The XRD patterns of sandstone were obtained using X’pert Pro MPD X-ray diffractometer made in Netherlands with measured temperature of 20°C and moisture of 65% whereas the XRD patterns of mudstone were finished using D/max-2500 X-ray diffractometer with measured temperature of 20°C and moisture of 50%. Preparation, analysis, and interpretation procedures are modified from Moore and Reynolds  and Hillier . Cathode luminescence (CL) analyses for 10 typical samples were performed using an Olympus microscope equipped with a CL8200-MK5 CL instrument.
Among all the analyses above, point counting, XRD of sandstones, SEM, and CL were done in Key Laboratory of China University of Geosciences, whereas XRD of mudstones and trace element analysis were carried out in Institute of Bohai Oilfield.
A series of diagenetic processes are identified in the Paleogene medium-deep reservoirs, Huanghekou Sag, including precipitation of carbonate, zeolite minerals and dissolution of feldspar, quartz, and zeolite minerals. Precipitation and dissolution of these minerals played a significant role in reservoir quality of the Paleogene medium-deep reservoirs. In addition, these diagenetic products developed in specified aqueous medium environment that can be effectively used to trace pore fluid properties, combined with analysis of trace elements and X-ray diffraction. Minor diagenetic phases (e.g., ferroan dolomite) are not depicted in this study because of leanly abundant through the Paleogene medium-deep intervals.
4.1. Precipitation and Dissolution of Zeolite
Discrete plot of mineral compositions based on XRD analysis of sandstone reservoirs indicates that zeolites occur mainly in the E3d1+2 interval with high content, which may be controlled by aqueous medium environment enriched ferromagnesian minerals (Figure 4). Similarly, precipitation and dissolution of zeolites are observed commonly in the E3d1+2 interval. In thin sections, zeolites occur as intergranular cements with prismatic perpendicular to basalt rock fragments filling intergranular pores (Figure 5(a)). In addition, some interesting phenomena are observed that zeolites replace volcanic fragments, feldspars, and quartz that line intergranular pores. Pore types of the E3d1+2 interval occur mainly as zeolite dissolution pore, amygdala dissolution pore based on thin section observation.
4.2. Dissolution of Quartz and Feldspar
Partial to extensive dissolution of detrital quartz grains was observed in the E2s1+2 interval (Figures 6(a), 6(b), and 6(c)). Based on microscope observations, two types of quartz dissolution can be identified: quartz grains dissolution occurred along the boundaries and interior of grains, with embayed boundary and vugular shape, respectively; quartz grains dissolution occurred along the internal fracture of grains. Dissolution of detrital feldspar grains is pervasively developed in the Paleogene reservoirs of study area. The predominant dissolved feldspars are K-feldspar grains (Figure 7(a)). Locally, dissolved plagioclase can be observed but is not common.
4.3. K-Feldspar Albitization
On the basis of environmental scanning electron microscope (ESEM) and backscattered electron (BSE) image analysis, albitization occurred mainly in the E2s1+2 interval. Albitization of K-feldspar is predominant the interest area, whereas albitization of plagioclase only can be observed locally (Figures 6(d), 6(e), and 6(f)). In addition, euhedral albite crystals are developed as pore-filling cements and as replacement of dissolved K-feldspar and plagioclase (Figure 6(e)). Albitization of K-feldspar is displayed commonly along cleavage planes of detrital feldspar grains (Figure 6(f)).
4.4. Carbonate Cements
Carbonate cements are the volumetrically predominant authigenic minerals in the E2s1+2 interval and mainly include calcite, dolomite, and ferroan dolomite. The CL micrograph of calcite cements displays a homogeneous orange luminescence color (Figure 7(c)). In thin sections, dolomite cements cause mainly ctenoid dolomite encrustation around the surface of detrital grains or partly epidermis oolite formed by engulfing quartz grains (Figures 7(d) and 7(f)). Ferroan dolomite is the most pore-filling cements remaining in intergranular pores which resulted in the pore connectivity becoming worse. Ctenoid dolomite encrustation adheres to the surface of detrital grains but inhabited the overgrowth of quartz grains which effectively preserved intergranular pore.
4.5. X-Ray Diffraction of Clay Mineral
Various types of clay minerals with different abundances occur in the Paleogene interbedded mudstones of sandstone reservoirs, as revealed by XRD, composed of illite, mixed-layer illite/smectite (I/S), kaolinite, chlorite, and mixed-layer chlorite/smectite (C/S). XRD analysis data of interbedded mudstones indicates that illite and mixed-layer illite/smectite are the dominating clay minerals in the whole Paleogene intervals except the E3d1+2 interval which may be associated with the eruption of basalt and the content of illite shows positive correlation with increasing burial depth, particularly, deeper than about 2700 m where it increases sharply (Figure 8). Mixed-layer chlorite/smectite mainly exists at depths shallower than 2800 m corresponding to the E3d1+2 interval and the upper part of E3d3 interval and shows decreasing trends with increased burial depth (Figure 8). The content of kaolinite is characterized by significant increase in the lower part of E3d3 interval between 2900 m and 3000 m with percentage ranging from 10 to 30 m. On the contrary, the percentage of mixed-layer illite/smectite decreases quickly deeper than 2800 m to 3000 m. Therefore, the contents of the two-clay minerals variation with depth show that obviously mirror trends occurred in the lower part of E3d3 interval (Figure 8).
4.6. Trace Element Distribution of Paleogene Mudstone
Trace element distribution of well BZ-1 between E2s1+2, E3d3, and E3d1+2 is characterized by three types of stepped reduction where boron concentrations decrease significantly. Based on analysis of trace elements, boron concentrations of E2s1+2 interval and E3d1+2 interval are the most and the least, respectively, and E3d3 interval is intermediate. Within the E2s1+2 interval, concentration of boron increases slightly with increased burial depth ranging from 32.6 to 46.6 μg/g with an average value of 38.16 μg/g (Figure 9). During the E3d3 interval, however, the content of boron reduces to level ranging from 15.4 to 33.9 μg/g with an average of 26.8 μg/g (Figure 9). For E3d1+2 interval, boron concentrations decrease to a lower interval ranging from 5.51 to 26.5 μg/g with an average value of 10.7 μg/g (Figure 9). Similar trend is shown in the ratios of B/Ga element, as revealed in Figure 9, but different in numeric value.
5.1. Evolution of Pore Fluids
The various diagenetic processes in the Paleogene medium-deep reservoirs in the Huanghekou Sag record the pore varying fluid properties. The typically diagenetic events, clay mineral compositions, and trace elements distribution characteristics discussed above demonstrate large difference in the pore fluid properties with different intervals during the burial process of the sandstone reservoirs accompanied by basalt eruption. Therefore, in an attempt to investigate these differences using XRD data and trace elements are necessary.
Different diagenetic events developed in various deposition environment with diverse pore fluid properties that effectively help us understand the pore fluid evolution . Based on thin section observation, as discussed above, typically diagenetic events developed in Paleogene sandstone reservoirs include precipitation and dissolution of zeolite, dissolution of quartz and feldspar, and K-feldspar albitization.
5.1.1. Ferromagnesian Aqueous Medium Environment
Zeolite is alkaline metal aluminum silicate mineral of framework structure essentially formed through the water-rock interactions of volcanic glass whether exogenous or endogenous origin . Previous studies demonstrated that zeolite may develop with various sedimentary environments but commonly formed during the medium-alkaline water environment with pH value ranging from 7 to 10 and generally related to alteration of volcanic tuff and volcanic glass [39–41]. Furthermore, zeolite concentrated on saline alkaline environment which provides the source of SiO2 and Al2O3 for the precipitation of zeolite. When formation water medium flows through the basalt strata, the released K+, Na+, Ca2+, Al3+, Fe3+, and Mg2+ of basalt resulted in the formation of alkaline water environment with increased pH and enrichment of ferromagnesian minerals which favors generation of zeolite. Thus, interaction of alkaline aqueous medium and volcanic glass leads to generation of zeolite minerals [40, 42–44]. Previous study based on electron microprobe analysis demonstrated that zeolites in well BZ-1 of the study area are mainly analcimes . Analcimes might not be directly formed by the alteration of volcanic glass but transformed by other zeolites (commonly clinoptilolites). Early formed zeolites (clinoptilolite) may react at shallow depths to form analcime, albite, and K-feldspar as a function of either age or salinity. The transformation of clinoptilolite to analcime can be idealized as the following reaction , but the actual chemical change is commonly more complex than this.Given that, it is well explained that high contents of zeolite based on thin section observations and XRD data occurred mainly in the layer (E3d1+2) containing basalt (Figures 4 and 5). With the influence of basalt eruption, ferromagnesian minerals of basalt in sandstone reservoirs of E3d1+2 dissolved led to development of ferromagnesian aqueous medium environments that benefits the formation of zeolite. However, E3d3 with no significant influence of basalt eruption is corresponding to less abundance of zeolite. This phenomenon is consistent with clay minerals dominated by mixed layers C/S in E3d1+2 (Figure 8), which can be interpreted as strong hydration and hydrolysis of tuff and eruptive rock detritus precipitated voluminous authigenic smectite at an early diagenesis stage [46–48]. Smectite can easily transform to mixed-layer I/S or discrete illite with increasing temperature where a potassium source exist ([49–52]; Cama et al. 2000). Meanwhile, the conversion of smectite to mixed-layer C/S occurred where aqueous medium environments were enriched in magnesium. Large amounts of Fe2+ and Mg2+ released due to intense hydrolization of unstable volcanic rock fragments that resulted in pore fluids rich in ferromagnesian provided the condition for the formation of chlorite. Therefore, that is why the content of mixed-layer C/S of E3d1+2 increases abruptly as shown by XRD analysis (Figure 8).
5.1.2. Alkaline Aqueous Medium Environment
For a long time, quartz is considered to be chemically stable mineral that barely participates chemical diagenetic reactions except quartz overgrowth. As the research goes, however, more and more scholars realized that the dissolution of quartz grains and quartz overgrowth occurred during diagenesis stage [53–59]. Experimental simulation also confirmed the dissolution of quartz occurred and, based on determinations of simulation experiment, experimental formula was obtained that quartz dissolution rates are a function of pH, temperature, and alkaline ions concentration . Although multiple factors can affect quartz dissolution, the predominant factor controlled quartz dissolution is pore fluid geochemical property [55, 59, 60]. Because of the stability of quartz, it is difficult to dissolve and commonly will become unstable in alkaline environments where dissolution and alternation is developed. Therefore, the dissolution of quartz, generally, indicates the alkaline aqueous medium environments as exemplified by many case studies [58–60]. The quartz dissolution in the study area is mainly observed in E2s1+2 (Figure 6). During E2s1+2, lacustrine basin experienced the arid climate of alkaline aqueous medium coupled with part hydrolysis of ferromagnesian-rich minerals, resulting in enrichment of sodium and potassium. With the enrichment of the alkaline ions, pH value of pore fluids became high. It thus can be concluded that the deposition aqueous medium environment may be alkaline during the period of diagenesis. In addition, local dissolution of detrital quartz grains can be observed in the E3d3 interval but is less common (Figure 6), which indicates that the aqueous medium environment was inferred to be affected by that of the E2s1+2 interval.
Similarly conclusion can be drawn by the study on the analysis of trace element. Trace element is substantially sensible parameter indicating the salinity change which can be used to trace the pore fluid aqueous medium condition and therefore to deduce the deposition environment [61–64]. The content of boron is positively correlated with the salinity, that is, the higher content of boron the higher salinity [32, 65]. As is shown by the distribution of trace element of well BZ-1, the content of boron displays three stages increased with progressively increasing burial depth between E3d1+2, E3d3, and E2s1+2 (Figure 9). This trend indicates that the paleosalinity of E2s1+2 is the highest, E3d3 is second, and E3d1+2 is the least. It is worth noting that the layer containing the lowest paleosalinity in E3d1+2 is well correlated with the layer containing widespread distribution of basalt in E3d1+2. Therefore, it is not difficult to deduce that, during the deposition of E2s1+2, lake basin is narrow and shallow with high salinity of alkaline aqueous medium environment. During the sedimentation of E3d3, the basin experienced rapid subsidence and resulted in increasing in water depth and development of braided river delta. As a consequence, strong inputs of terrestrial deltaic fresh water significantly accelerated the pore fluids dilution causing decrease in salinity. However, braided river delta is not developed in E3d1+2 which experienced weak inputs of terrestrial fresh water. Hence, the lowest paleosalinity in E3d1+2 might not be the cause of input of terrestrial fresh water but interaction of pore fluids and basalt. Volcanic activities might have influenced the pore fluid salinity and further research is needed to understand the mechanism how volcanic activities influence the pore fluid salinity. Another parameter can be used to reveal that the paleosalinity is the ratios of B/Ga that display the corresponding trend with the content of boron. All of this reveals that the pore fluid aqueous medium environment of E2s1+2 is alkalinity with high salinity. Besides that, the valuable information for the pore fluid aqueous medium environment of sandstone reservoirs can be provided by clay minerals composition. During the period of sedimentary process of E2s1+2, lake water was saline associated with arid climatic environments that favored enrichment of potassium and, with the increasing burial of depth as well as the increase of temperature and press, kaolinite transforms to illite based on XRD data.
The albitization of detrital K-feldspar and plagioclase is one of the most important diagenetic changes occurring in sandstone reservoir [66–68]. Many previous studies documented that the albitization of detrital K-feldspar commonly occurred in the alkaline aqueous medium environment [60, 69]. The presence of detrital K-feldspar albitization is observed mainly in E2s1+2 and also can be found in E3d3, but less common, indicating the alkaline aqueous medium environment during the sedimentary process of E2s1+2 as well as the weak alkaline condition of E3d3.
In summary, based on the analysis of petrologic characteristics and pore fluid properties, evolution of pore fluids was investigated and three of distinguished stages are identified. During E2s1+2, lacustrine basin experienced the arid climate of alkaline aqueous medium coupled with part hydrolysis of ferromagnesian-rich minerals, resulting in enrichment of sodium and potassium with the highest content of boron and predominant clay minerals are potassium-rich illite and mixed-layer I/S (Figures 8, 9, and 10). During E3d3, influx of fluvial fresh water together with hydrolysis of ferromagnesian mineral developed mixing aqueous medium environment diagenetic model (Figure 10). Mixing aqueous medium pore fluids in E3d3 are characterized by interaction of pore fluids in both E2s1+2 and E3d1+2 and diagenetic processes exhibit integrated characteristic of both E2s1+2 and E3d1+2. The composition of the primary clay minerals is illite and mixed-layer I/S which may be caused by interaction of potassium-rich brine. During E3d1+2, strong hydrolysis of unstable ferromagnesian mineral and weak influx of fluvial fresh water resulted in the pore fluids enriched in ferromagnesian minerals and ferromagnesian aqueous medium environment diagenetic model developed due to volcanic activities (Figure 10).
5.2. Factors Controlling the Reservoir Quality
5.2.1. Early Precipitation of Zeolite Associated with Basaltic Eruption
The pore fluids flow through the basalt strata dissolving the Fe2+ and Mg2+ of the igneous rocks to form a ferromagnesian aqueous medium environment where zeolite is precipitated in the intergranular pores. The presence of zeolite precipitation increases the resistance to compaction favoring the preservation of primary pores. However, during the thermal evolution stage of the source rocks, dissolution of zeolite developed due to expulsion of organic acid forming secondary pores (Figure 5). Therefore, it is easily inferred that proximal sandstone formations combined with the influence of strong hydrolysis of ferromagnesian minerals commonly are favorable for the development of sandstone porosity, such as transition zone of braided river delta development zone and basaltic eruption zone.
5.2.2. Impacting of Compaction on Reservoir Quality
A large number of exploration practices show that compaction is the main factor leading to the reduction of primary porosity of sandstone reservoirs. Therefore, the effective inhibition of compaction can significantly affect the preservation of pores and improve the reservoir quality. The volcanic rocks are widely distributed in the study area and vertical distribution is mainly focused in E2s1+2 and E3d1+2. During the volcanic eruption stage, volcanic conduit facies and overflow facies constructed I-shaped volcanic structure which formed roof support effect weakening the compaction of overlying formation . Therefore, the primary pores can be substantially preserved improving the reservoir physical property.
5.2.3. Inhibition of Quartz Overgrowth and Dissolution of Detrital Quartz Grain
Quartz overgrowth is considered the main factor that reduced the porosity of medium-deep reservoirs and commonly 1% of quartz overgrowth resulted in 10% of the pore loss . Therefore, the pore preservation of medium-deep reservoirs can be achieved by inhibiting the chemical compaction of quartz. In the study area, the quartz grain surface observed the presence of ctenoid dolomite encrustation developed under the alkaline aqueous medium environment, effectively inhibiting the overgrowth of quartz (Figure 7(d)). As a result, the pores can be better preserved. As is shown by the porosity data in well BZ-1, there is no significant decrease in porosity with progressively increasing burial depth between E3d1+2, E3d3, and E2s1+2 (Figure 3). Furthermore, secondary pores produced resulted from the dissolution of detrital quartz also found based on thin section observation which improved the quality of reservoir (Figures 6(a), 6(b), and 6(c)).
5.3. Implications for Petroleum Exploration
The study has broad implications for exploration and development in other deeply buried sandstone reservoirs, especially but not exclusively related to volcanic activities. Sandstone reservoirs with development of volcanic rocks have not been considered prime potential hydrocarbon reservoirs due to poor reservoir quality. The present study, however, demonstrated that ferromagnesian-rich pore fluid aqueous medium environment resulted from volcanic activities developing typical diagenetic events such as precipitation and dissolution of zeolite associated with basaltic eruption effectively improving the reservoir quality. In turn, related diagenetic processes record the pore fluid property which provides us with new insight into investigating the pore fluid evolution and predicting the reservoir quality. Meanwhile, volcanic conduit facies and overflow facies constructed I-shaped volcanic structure which increased resistance but reduced compaction of strata, preserving the primary pores and improving the reservoir quality.
Pore fluid evolution influenced by volcanic activities in sandstone reservoirs has a critical impact on diagenetic processes which controlled the reservoir quality, as exemplified by deeply buried sandstone reservoirs in the Huanghekou Sag, Bohai Bay Basin, China. On the basis of our studies, the following conclusions can be drawn.(1)Volcanic activities exert a significant influence on pore fluid properties depending on volcanic activity intensity. Evolution of pore fluids can be divided into three stages corresponding to volcanic activities: ① E2s1+2: alkaline aqueous medium environment is developed with insignificant influence by volcanic activities, which can be interpreted as alkaline lake basin aqueous medium diagenetic model; ② E3d3: affected by influx of fluvial fresh water together with hydrolysis of ferromagnesian mineral, mixing aqueous medium environment occurred and volcanic intereruptive period diagenetic model was established; ③ E3d1+2: multiphase volcanic activities resulted in the formation of ferromagnesian aqueous medium environment and development of basal eruption diagenetic model.(2)The vertical distribution of pore fluids with varied properties in different intervals controls the diagenetic processes and reservoir quality. ① Alkaline aqueous medium pore fluids formed in E2s1+2, causing quartz dissolution, K-feldspar albitization, and dolomite encrustation. ② Ferromagnesian pore fluids in E3d1+2 resulted in zeolite precipitation in early diagenesis and zeolite dissolution during thermal evolution stage due to expulsion of organic acid. In addition, the predominant clay mineral is mixed-layer C/S influenced by hydrolysis of unstable ferromagnesian-rich mineral resulting from volcanic activities. ③ Mixing aqueous medium pore fluids in E3d3 are characterized by interaction of pore fluids in both E2s1+2 and E3d1+2. Therefore, the diagenetic processes exhibits integrated characteristic of both E2s1+2 and E3d1+2.(3)A series of controlling factors that played positive effect in the reservoirs quality are concluded including dolomite encrustation inhibited quartz overgrowth developments, quartz and zeolite dissolution formed secondary pores, and I-shaped igneous structure increased the resistance of compaction.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This research was funded by the National Natural Science Foundation of China (no. 41572084) and the National Science and Technology Major Project (no. 2016ZX05024-003-007). Thanks are due to the Tianjin Branch of the China National Offshore Oil Corporation for providing data used in this study and the permission to publish the results.
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