Introduction
After more than 30 years of exploration of the Cenozoic igneous rocks in the Bohai Bay Basin, some reservoirs dominated by basic rocks have been discovered, such as eruptive rock, intrusive rock, and intrusive contact metamorphic rock, in the Paleogene Shahejie Formation. On the one hand, compared with the preCenozoic volcanic rocks in hydrocarbon-bearing basins, the Cenozoic volcanic rocks in the Bohai Bay Basin have been exposed to the surface only for a short period of time, and generally not suffered from large-scale weathering and leaching. The physical properties of the volcanic reservoirs are generally poor so that only smaller oil reservoirs may develop [1⇓-3]. On the other hand, deep magma rising into shallow formations became shallow intrusions of different sizes. These intrusions affected sedimentary host rocks in ways of thermal contact metamorphism, hydrothermal dissolution and extrusion transformation,and resulted in reservoirs of contact metamorphic aureole and metamorphic-lithologic reservoirs with certain scale around the intrusions [4]. By the end of 2023, a lot of reservoirs in contact metamorphic aureole had been discovered in the Paleogene formations in the Caojiawu and Caiyu areas in the Langgu sag, the Linyi and Xiakou areas in the Huimin sag, the Chunxi area in the Dongying sag, the Luojia area in the Zhanhua sag, and the Zhangdong area in the Qikou sag in the Bohai Bay Basin. In addition, reservoirs in contact metamorphic aureole had been discovered in the northern slope of the Gaoyou sag and the Qintong sag in the Subei Basin, showing good exploration prospects [4⇓⇓⇓⇓-9].
Foreign scholars began to study the impact of shallow magmatic intrusion on clastic host rocks in the 1980s. Through outcrop observation, they studied the impact of some "meter-scale" thin intrusions on the host rocks from the aspects of petrology and diagenesis, and pointed out that the host rocks adjacent to the contact surface of the intrusions were affected by thermal contact metamorphism, resulting in a "contact metamorphic aureole" [10]. Remote sensing, 3D seismic survey, controlled source electromagnetic or magnetotelluric survey, and outcrop observation were used to describe the distribution of intrusions, and logging data including gamma ray, density, acoustic and resistivity curves were used to identify and classify the intrusions and their contact metamorphic aureoles [11⇓-13]. Due to the small size and shallow burial, these exposed intrusions have limited effects of thermal metamorphism and hydrothermal dissolution on the host rocks. Meanwhile, it is believed that the physical properties of the contact metamorphic aureole in highly porous unconsolidated and consolidated sandstone around thin dikes have been deteriorated or have no significant changes, while the physical properties of the contact metamorphic aureole into the mudstone around sills have been significantly improved due to the development of intrusion-related fractures, structural fractures and dissolution vugs [14⇓⇓⇓⇓⇓⇓-21].
Domestic scholars also started to study the intrusions and reservoirs of contact metamorphic aureole in the 1980s. Through core observation, organic geochemistry and 3D seismic survey, they studied the characteristics and the development mechanism of reservoirs in contact metamorphic aureole generated by some thick intrusions in adjacent source rocks. Some scholars believed that the reservoir physical properties of metamorphic rock was significantly better than that of intrusions, and established a method for identifying intrusion and contact metamorphic aureole, as well as a reservoir space model[4⇓⇓⇓⇓-9]. Previous studies on the reservoirs of contact metamorphic aureole mainly focused on field outcrop description of meter-scale intrusions and geochemical analysis of thick intrusions downhole, emphasizing the influence of hydrothermal processes on high porosity-permeability sandstones and intrusive compressions on mudstones, but neglecting the important influence of thermal baking metamorphism on the consolidation and preservation of pore spaces in tight reservoirs. Additionally, there is a lack of systematic research on the type and formation mechanism of reservoir spaces in reservoirs of contact metamorphic aureole.
Our study proposed a geological model of igneous intrusion contact metamorphic system after systematically summarizing the research results of this type of reservoirs at home and abroad [4⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓-21], and taking the Paleogene Shahejie Formation in the Nanpu sag as an example. Based on this model, this paper conducted research on intrusions, contact metamorphic aureole and metamorphic reservoirs in order to provide a reference for promoting the discovery of exploration targets of deep contact metamorphic rock in Nanpu sag and other hydrocarbon-rich sags.
1. The overview of the study area
The Nanpu sag is one of the sags with the most developed igneous rocks in the Bohai Bay Basin, with an area of 1 930 km2. The sedimentary strata are successively the third, second and first members of the Paleogene Shahejie Formation (referred to as Sha3, Sha2, and Sha1 members), the third, second and first members of the Paleogene Dongying Formation of the System (referred to as Dong3, Dong2, and Dong1 members), and the Neogene Guantao Formation, Minghuazhen Formation, and Quaternary from bottom to top [22]. The Cenozoic basic igneous rocks are distributed from Shahejie Formation to Guantao Formation, while the area of the volcanics is about 1 372 km2, accounting for 71% of the total sag area. It is mainly distributed in Guantao Formation and the top of Dong1 Member in the vertical direction, and widely distributed in the plane (Fig. 1 ). The area of intrusions is about 796 km2, and accounting for 41% of the total sag area. In the vertical direction, it is mainly distributed in the Shahejie Formation and Dongying Formation, including 27 intrusions developed from Dong3 to Sha3 members, with Dong3 Member having the widest distribution range and other layers scatteredly distributed, and it is mainly in the south and center of the Nanpu sag on the plane (Fig. 1 ). The largest intrusion in Dong3 Member of the Laoyemiao belt covers 145 km2 and is about 50 m thick. The thickest intrusion in Sha3 Member of Well Nanpu 280, called No. 1 intrusion, is about 110 m thick and covers 25 km2. There are three sets of intrusions developed in the Shahejie Formation of Nanpu 280 Well region vertically. This paper focuses on the systematic study of No. 1 intrusion. Early researches did not recognize the existence of metamorphic reservoirs associated with intrusions, and no hydrocarbon discoveries were made. In 2019, systematic core observation and sampling analysis were conducted on cored wells encountered with igneous rocks in the Nanpu sag. It was found that the characteristics of the associated reservoirs of this set of igneous rocks are similar to those of intrusive contact metamorphic aureoles in other areas of the Bohai Bay Basin. The thick basalt recorded in the mud logs was diabase at first, while the overlying andesite and rhyolite were contact metamorphic rocks caused by baking metamorphism in fact. In the same year, the PetroChina Jidong Oilfield Company discovered new oil layers when reviewing the well logging data of this set of metamorphic reservoir, and obtained high-yield industrial oil and gas flows at 36.3 t/d of oil and 4×104 m3/d of gas during well test. Subsequently, the research and exploration of deep reservoirs in contact metamorphic aureoles associated with intrusions have received increasing attention.
Fig. 1. Distribution of shallow intrusions (a) and histogram of igneous rocks development interval (b) in Dongying and Shahejie formations in Nanpu sag. |
2. Igneous intrusion contact metamorphic system model and key elements
During the Cenozoic, deep magma in the Bohai Bay Basin rose along active deep faults, and occurred as intrusions in areas where the intensity of fault activity and kinetic energy of magma decreased. Most of the intrusions distributed along the faults are dikes, which are generally narrow and distributed on the fault plane. The intrusions along layers are sills, which are generally 30-150 m thick and cover 20-150 km2, and have a large contact area with the host rock. The thick sills have obvious thermal contact metamorphism effect on the host rock, usually resulting in thermal contact metamorphism (at normal temperature), baking metamorphism (at high temperature), and contact metasomatic metamorphism [21]. The lithological and structural characteristics of contact metamorphic aureoles of Bohai Bay Basin confirmed by a large number of wells indicate that the reconstruction of deep sills on sedimentary host rocks is reflected by thermal contact metamorphism and baking metamorphism, and the former is more common. However, the metamorphic aureole near the contact surface is prone to suffer from intensive baking metamorphism, so that organic matter fading and strong baking edges are common. The dikes formed on the fault plane are difficult to develop obvious metamorphic aureoles because they are narrow and contacted with different strata vertically with large temperature variation. In the field of hydrocarbon exploration, limited by effective data and research methods in the past, intrusions and contact metamorphic aureoles are often studied as separate geological bodies, so that understandings of their lithology, lithofacies, and even genesis are significantly different. Based on systematic study of available drilling data, it’s found that there is inherent genetic relationship and spatial symbiosis between intrusions and contact metamorphic aureoles in the Bohai Bay Basin, and a geological model has been established for the igneous intrusion and contact metamorphic system.
2.1. Igneous intrusion contact metamorphic system model
2.1.1. The connotation of the igneous intrusion contact metamorphic system
A large amount of high-temperature magma intruding into the sedimentary formations would induce thermal baking effects on the adjacent host rock, and form contact metamorphic aureoles in the slow condensation process [23]. Contact metamorphic aureoles stem from intrusions, and have a clear inherent genetic relationship and spatial symbiosis with the intrusions. At the same time, the thick intrusive magma undergoes regular changes in mineral crystallization and texture during condensation, forming palimpsest textures, low-to-moderate temperature metamorphic minerals, and low-to-moderate thermal metamorphic reservoirs during thermal contact metamorphism of the host rock. The igneous intrusion contact metamorphic system is defined as a system that encompasses intrusion, contact metamorphic aureole, normal sedimentary strata and metamorphic reservoirs, which are interrelated geological elements. The system consists of five structural layers, namely intrusion, upper metamorphic aureole, lower metamorphic aureole, normal sedimentary layers on the roof and floor. It also has three key elements: intrusion, contact metamorphic aureole (including metamorphic reservoir) and normal sedimentary layers (Fig. 2 ). This system takes intrusion and contact metamorphic aureole as a symbiotic system, therefore reducing the ambiguity of lithological interpretation, textural diversity and complexity of physical properties when studying contact metamorphic aureole alone.
Fig. 2. The model of the Cenozoic intrusion contact metamorphic system in the Bohai Bay Basin. N1g—Neogene Guantao Formation, E3d—Paleogene Dongying Formation, E2-3s—Paleogene Shahejie Formation. |
2.1.2. Logging response of five vertical structural layers
According to the model of the igneous intrusion contact metamorphic system, it can be divided into five layers of structure in a single well: intrusion, upper metamorphic aureole, lower metamorphic aureole, normal sedimentary layers on the roof and floor based on core observation and analysis results and rock electrical characteristics. Fig. 3 shows the classification results of Well NP280 in No. 1 intrusion and Well Zh 22 in Qikou depression.
Fig. 3. The classification of vertical layer structure of the intrusion contact metamorphic system in Nanpu-Qikou sags of the Huanghua depression, Bohai Bay Basin. |
(1) Intrusion. The lithology of the intrusion is gabbros-diabase, and acts as a “heat source” of the contact metamorphic aureole, characterized by low gamma ray (GR, 30 API), high density (2.73-2.82 g/m3), high velocity (5 200-5 560 m/s), and high resistivity (57-500 Ω•m). Low GR and high velocity are key features for lithological identification. Large variation in resistivity is influenced by differences in magma composition, crystallization degree and fracture development. (2) Upper and lower contact metamorphic aureoles. The upper and the lower metamorphic aureoles are subject to the thermal effect from the same “heat source”, so they have similar macroscopic, microscopic and electrical characteristics. The upper metamorphic aureole is generally slightly thicker than the lower due to the effect of thermal rise. Compared to normal sedimentary strata, they have higher GR (100-120 API), higher velocity (4 390-5 040 m/s), higher density (2.47-2.63 g/m3), and higher resistivity (15-42 Ω•m). The velocity curve shows a typical "step-like" shape that decreases gradually to the normal velocity from the contact surface towards the normal layers with decreasing baking intensity. The density varies significantly, and locally low due to pore and fracture development (e.g., relatively low density near the intrusion in Well Zh 22).
The resistivity is generally higher than normal sedimentary layers (10-20 Ω•m). Near the contact surface, there are spotted and strongly metamorphic slates, while away from the contact surface, there are mainly metamorphic sandstone and mudstone. (3) Normal sedimentary layers on the roof and floor. These layers are outside the upper and the lower contact metamorphic aureoles, and have not been affected by the thermal baking of intrusions. There developed typical sedimentary siltstone and mudstone with normal electrical characteristics located in the deltaic subfacies of the lower Sha3 Member. Usually, they belong to the same formation under the same or similar sedimentary environment, but are separated into two sets of strata intervals by the intrusions. The electrical characteristics of typical five structural layers provide important evidences for identifying the No. 1 intrusion and contact metamorphic aureoles.
2.2. Key elements
There are three basic elements in the intrusion contact metamorphic system. The first one is the intrusion. The Cenozoic igneous intrusions are dominated by basic rocks in the Bohai Bay Basin [1], which are almost layered sills 30-150 m thick and with central and marginal facies. The contact surfaces are often surrounded by xenoliths from the host rocks with fast-cooling edges dominated by vitreous material (approximately 10-30 cm). Usually, intrusions are not reservoirs, but can act as barriers. The second one is the contact metamorphic aureole (including metamorphic reservoir). The main body of the contact metamorphic aureoles is the result of weak-to-moderate metamorphism, and only the contact surface suffered from strong metamorphism, resulting in strongly baked edges and organic matter fading to grayish white (approximately 10-40 cm thick). At the same time, hydrothermal activities within the metamorphic aureole are relatively active, often flowing and circulating along the bedding plane from the main body to the end in the temperature-decreasing direction. Hydrothermal activities produce significant dissolution vugs. In addition, magma intrusions create various compression fractures in metamorphic aureoles (Fig. 2 ). The third one is the normal sedimentary strata. The strata outside the metamorphic aureole and not affected by contact metamorphism are the important basis for determining the lithology and physical properties of the host rock of the metamorphic aureole. The host rock of the metamorphic aureole can be sedimentary rock or others, and can be studied by the similar method used for similar lithologies, which will not be described in this paper.
2.2.1. Intrusion
Basic igneous intrusions often occur in the form of sills within sedimentary strata in a large area. They usually have the following three basic characteristics. There are often xenoliths of host rocks on the upper and lower edges. Longitudinally, the texture and mineral crystallization show systematic variations from the center to the contact surface, with the vitreous portion in the marginal facies being vitreous fine-grained diabase. Macroscopically, they often intersect with isochronous framework strata at low angles.
2.2.1.1. Indicative xenoliths at the top of intrusion
The occurrence of xenoliths at the top of igneous rock is an important evidence for determining intrusions. "Xenoliths" refer to geological phenomena where rock fragments from surrounding formations are incorporated into the igneous intrusion [24]. When xenoliths are found at the top of igneous rock, it indicates that the overlying host rock was entrapped in the flowing magma of the underlying intrusion, constituting a significant feature of the intrusive facies. In contrast, after magma erupts onto the ground, its surface rapidly becomes cool and solidified, and the following sediments are unable to penetrate the consolidated volcanic rock and form xenoliths.
There are three wells where continuous cores can be taken from the top of the No. 1 intrusion and its host rocks. Multiple layers of xenoliths, approximately 2-3 cm thick, were observed at the top of the igneous rock in Well NP203-50 located at the middle to end of the intrusion (Fig. 4 ). The upper edge of the core photo represents the contact surface between the intrusion and the host rock. The upper two-thirds of the photo depicts the xenoliths, while the lower dark part corresponds to the vitreous fine-grained diabase at the edge of the intrusion. These xenoliths exhibit the following four characteristics: (1) They have the same lithology as the overlying host rock and show sedimentary beddings, indicating that they are rock fragments from the overlying sedimentary host rocks that were incorporated into the intrusive magma, and the space among the fragmented rocks was filled with vitreous igneous rock at the edge of the intrusion; (2) The xenolith layer is generally parallel to the host rock, and the rock is relatively intact, suggesting that the overlying rock were engulfed by the intrusion at a later time, and didn’t undergo long-distance transportation and extrusion, so their angular shapes well retain; (3) The arrangement of the xenolith rocks exhibits certain kinematic characteristics, reflecting the direction of magma carrying the xenoliths; (4) The xenolith shows a similar spotty palimpsest texture to that of the host rock, revealing that the xenolith also suffered from magma thermal baking and contact metamorphism during transportation.
Fig. 4. Photos of the core and thin section of the xenolith at the top of No. 1 intrusion. (a) The arrow points to the center of the intrusion, 2/3 of the upper and middle are xenoliths, and the lower part is a large amount of vitreous diabase, 4 173.64 m, Well NP203-50, core photo; (b) clear characteristics of xenolith and the palimpsest texture of the baked spots, section corresponding to the green box in Fig. a. |
2.2.1.2. Regular changes of rock texture and mineral crystallization from center to edge
According to the changes of rock texture and mineral crystallization, an intrusion can be divided into two facies: central facies and marginal facies. If an intrusion is about 100 m thick, its marginal facies is typically around 5 m thick. Under the microscope with the same magnification, from the central facies to the marginal facies, the rock texture shows a regular change from a sequence of gabbros-diabase-intergranular-interstitial textures [25-26] (Fig. 5 ). In the central facies where the magma temperature is the highest and declines slowly, the conditions are favorable for mineral crystallization and growth, resulting in well-developed mineral crystallization. This facies typically exhibits the gabbros and gabbros-diabase textures composed of large-grained plagioclase phenocrysts and pyroxene phenocrysts. At the central facies 13 m from the intrusion boundary, plagioclase and pyroxene phenocrysts are coarse, and develop into a gabbros texture. Toward the marginal facies, it is dominated by diabase. The magma temperature at the marginal facies decreases rapidly, which is not conducive to mineral crystallization and growth. Plagioclase phenocrysts become smaller, pyroxene phenocrysts decrease, and there appear more vitreous matters with cooled edges near the contact surface, with intergranular and interstitial textures (Fig. 5a ). At the marginal facies 2.75 m from the intrusion boundary, the plagioclase and pyroxene phenocrysts become smaller, and the plagioclase phenocrysts are more than the pyroxene phenocrysts, with dominant intergranular-interstitial textures (Fig. 5b ). Near the contact surface between the intrusion and the host rock, the plagioclase phenocrysts become smaller and less, a large amount of vitreous matters appear, with vitreous interstitial texture dominated (Fig. 5c ). Despite being frequently misidentified as basalt by thin-section microscopy, this rock is correctly classified into vitreous fine-grained diabase when considering the characteristics of the cooled edges. It is evident that the rock texture and phenocryst variation from the central facies to the marginal facies of the No. 1 intrusion conform to the facies sequence, rock crystal and texture change laws of basic intrusions.
Fig. 5. Rock texture and mineral crystal variations from the central facies to the marginal facies of No. 1 intrusion. (a) Central facies of gabbros with a gabbros-diabase texture, large crystals of pyroxenes and plagioclases, Well NP280-41, 4 204.60 m, orthotropic polarization; (b) marginal facies of gabbros with an intergranular texture, smaller crystals of plagioclases, Well NP203-50, 4 177.19 m, orthotropic polarization; (c) marginal facies of vitreous gabbros with an interstitial texture at the edge, small crystals of plagioclases distributed in the direction of magma flow dispersed in a large amount of altered vitreous material, orthotropic polarization oriented thin sections and gypsum test plates, Well NP203-50, 4 174.64 m. Aug—pyroxene; PI—plagioclase. |
2.2.1.3. Igneous rock penetrating the sedimentary strata at a low angle
Basic intrusions commonly penetrate the sedimentary strata at a low angle, especially in the Dongying Formation. For example, the intrusions of Dong3 Member in the Laoyemiao belt penetrate into the normal sedimentary strata at 16°. No. 1 intrusion is deep and near No. 2 fault, so the seismic responses of the host rock are unclear. It can be roughly seen that it obliquely intersects with the host rock at 10°. When basalt of eruption facies has not experienced a sedimentary hiatus caused by large tectonic movements, it is generally parallel to the underlying strata, and its top often exhibits a hummocky shape, such as the parallel contact between overflow basalt of the Guantao Formation and the underlying Dongying Formation. Another characteristic of layered intrusions is that there is a distinct "upward tilt” feature at both ends on the seismic profiles, caused by the weakening of the dynamic force at the front of the magma intrusion and encountering lithological, physical property and occurrence changes. This low-angle penetration phenomenon can assist in macroscopic identification of intrusions.
Based on the analysis of xenoliths in igneous rock, facies division, the characteristics of mineral crystallization, regular texture variation, reasonable naming of rocks, and macroscopic oblique penetration phenomena, it is determined that No. 1 igneous rock is a typical basic intrusion.
2.2.2. Contact metamorphic aureoles
In the intrusion contact metamorphic system, if the same set of strata is "separated" by intrusions, the upper and lower host rocks are generally similar, and the upper and lower metamorphic aureoles are almost similar in characteristics, too. In the upper metamorphic aureole of No. 1 intrusion, oil and gas discoveries have been made. According to the cores from three wells, the host rock and metamorphic characteristics of the upper metamorphic aureole are as follows.
2.2.2.1. The host rock of the metamorphic aureole is sedimentary rock
For most sedimentary rocks, the rock type can be determined by depicting the rock texture and sedimentary structure through core observation. The observation of core and thin section from coring interval with 12.6 m length from the overlying formation adjacent to No. 1 intrusion reveals typical sedimentary rocks except for three thin layered intrusions. Fig. 6a and Fig. 6b show core photographs taken at 2.75 m and 3.00 m from the contact surface respectively, depicting mudstone with horizontal fine beddings, and displaying spotted and less metamorphic mudstone under the microscope. The horizontal fine beddings, composed of alternating dark and light gray stripes, indicate the host rock may be composed of silty mudstone deposited in a shallow to semi-deep lake environment. Fig. 6c shows the photograph of a core taken at 6.06 m from the contact surface, which is fine siltstone with horizontal beddings, and spotted metamorphic siltstone containing hot metamorphic minerals under the microscope. In addition, some biological debris were observed, and all of them indicate that the host rock of the contact metamorphic aureole overlying No. 1 intrusion is sedimentary rock deposited in a semi-deep lake environment.
Fig. 6. Photos of metamudstone samples taken in the upper metamorphic aureole of No. 1 intrusion. (a) Light gray metamorphic mudstone whose host rock is mudstone with horizontal fine beddings, and 2.75 m from the upper contact surface, Well NP203-50, 4 171.89 m; (b) dark gray metamorphic mudstone whose host rock is mudstone with horizontal fine beddings, and 3.00 m from the upper contact surface, Well NP203-50, 4 171.64 m; (c) brown, oil-immersed, metamorphic and fine siltstone whose host rock is siltstone, and 6.06 m from the upper contact surface, Well NP203-50, 4 168.58 m. |
2.2.2.2. Typical palimpsest texture
The sedimentary formation adjacent to the intrusion has obvious thermal baking metamorphism, with thermal baking metamorphic spots and thermally metamorphosed minerals superimposed on the sedimentary host rock, therefore forming a palimpsest texture. Only in the metamorphic aureole near the contact surface, there are some characteristics similar to crystalloblastic texture with a certain thickness and displaying high metamorphism. These palimpsest texture and thermally metamorphosed minerals are typical features of thermal contact metamorphism.
Palimpsest texture refers to the new metamorphic minerals from the cements and matrix among some original particles after crystalloblastic and recrystallized processes, and still retains some or most of the texture and mineral characteristics of the host rock [27]. Palimpsest texture contains both metamorphic phenomenon and metamorphic minerals, and retains the texture and mineral characteristics of the host rock. Crystalloblastic texture is the result of recrystallization and metamorphic crystallization during the metamorphic process of the host rock [27]. Generally, the rock is holocrystalline with high metamorphism and no visible features of the original rock. This phenomenon mostly occurs near the contact surface. Based on thin section identification of multiple core samples, we found that the contact metamorphic aureole of No. 1 intrusion mainly developed palimpsest argillaceous texture, palimpsest silty texture, and palimpsest bedding texture (Fig. 7a-7b ).
Fig. 7. Photos of palimpsest texture and palimpsest bedding texture and typical metamorphic minerals in the upper metamorphic aureole of No. 1 intrusion. (a) Light spots and sedimentary beddings in metamudstone constitute palimpsest texture and palimpsest bedding texture, Well NP280-41, 4 186.22 m, single polarization; (b) light spots in metamorphic fine silty mudstone, palimpsest texture with sedimentary beddings, Well NP203-50, 4 173.04 m, single polarization; (c) cordierite and chiastolite in metamorphic fine silty mudstones in the upper metamorphic aureole, Well NP203-50, 4 166.96 m, single polarization (100X); (d) cordierite with six clear crystal faces, Well NP203-50, 4 166.96 m, orthorhombic polarization (200X); (e) andalusite, garnet, calcite, and other thermally metamorphic minerals in silty mudstone in the lower metamorphic aureole, Well NP280-41, 4 272.20 m, orthorhombic polarization (100X) and gypsum test plate; (f) thermally metamorphic minerals such as andalusite, garnet and chiastolite in silty mudstone in the lower metamorphic aureole, Well NP280-41; 4 273.67 m, orthorhombic polarization (50X). Chs—chiastolite; Crd—cordierite; Grt—garnet; And—andalusite. |
For palimpsest argillaceous texture and palimpsest bedding texture in metamudstone, the host rock has clear horizontal micro-beddings, the clay minerals alternate with small clastic particles, and there are clearly outlined light-colored epigenetic spots distributed along the laminate of fine clastic grains (Fig. 7a ). The spots formed by thermal baking metamorphism generally appear in the laminate of fine clastic grains less than 0.01 mm, forming palimpsest argillaceous texture (Fig. 7b ). For the palimpsest silty texture in the metamorphic fine siltstone stripes, the host rock is fine siltstone stripes with different grain sizes alternating with sedimentary beddings, and superimposed with epigenetic spots after metamorphism. The bright layers are composed of fine thermally metamorphic minerals such as cordierite and chiastolite, and the spots in the stripes are also composed of well-crystallized and thermally metamorphic minerals. In addition, there are other palimpsest textures such as palimpsest bioclast, palimpsest ring-shaped reaction rims, metasomatic residues and stripes.
2.2.2.3. Various metamorphic minerals at medium to low temperature
Thermally metamorphic minerals in the contact metamorphic aureole are the direct products of thermal contact metamorphism of the host rock. According to the combination of thermally metamorphic minerals, it is believed that intermediate and low-grade metamorphic minerals were developed in the metamorphic aureole of No. 1 intrusion, but no high-grade metamorphic minerals. The upper metamorphic aureole is an association of chiastolite and cordierite minerals, while the lower metamorphic aureole is an association of andalusite, garnet and fushanite minerals. In addition, there are two types of thermally metamorphic minerals with different crystal sizes in the upper metamorphic aureole: (1) Large crystalline thermally metamorphic minerals whose particles generally range from 1 mm to 3 mm, and which are significantly different from the matrix, with some edges showing the outlines of chiastolite and cordierite. The internal crystallinity of the particles is poor, with a cryptocrystalline structure, and there is almost no optical activity under orthogonal polarization. The crystal outline is relatively clear under single polarization; (2) Small crystalline thermally metamorphic minerals, mostly less than 0.1 mm, constitute the matrix of the rock. Under 100 times of single polarization, small particles have clear outlines of chiastolite and cordierite, with good crystallinity, and obvious chiastolite crystal nuclei (Fig. 7c ). Under 200 times of orthogonal polarization, the crystal outline is clearer, and six extinction crystal faces of cordierite have symmetrical extinction characteristics. Some chiastolite can be seen with four extinction crystal faces (Fig. 7d ). Typical thermal metamorphic minerals are also observed in the lower metamorphic aureole, with good crystallization and clear optical characteristics. Chiastolite shows four crystal faces that are symmetrically extinct. Garnet shows four extinction crystal faces. The longitudinal section of long-banded andalusite shows a first-order interference color, parallel extinction, and alternating distribution with garnet (Fig. 9e-9f ). In addition, a large amount of laumontite was observed under an electron microscope.
From the development of thermally metamorphic minerals in the upper metamorphic aureole, in Well NP280 close to the intrusion conduit, the mineral particles are relatively large and poorly crystallized at 3.8 m from the contact surface. However, in Well NP203-50 drilled in the central part of the intrusion, the thermally metamorphic mineral particles are smaller, well crystallized with obvious optical features at 7.7 m from the contact surface. Can that crystallization changes with the vertical distance from the contact surface of the intrusion suggest that a suitable temperature location from the intrusion is more conducive to the growth of metamorphic mineral crystals?
2.2.2.4. Medium to low-grade thermally metamorphic rocks
The contact metamorphic aureole of No. 1 intrusion generally experienced medium to low metamorphism, resulting in the development of palimpsest texture and palimpsest bedding structure, but no obvious crystallographic structure. Most of the palimpsest texture after baking is dominated by the original texture, while only a portion of the metamorphic aureole close to the contact surface shows no original texture in the palimpsest texture. The lithology of the metamorphic aureole is mainly divided into two categories: (1) The relatively highly metamorphic spotted slate has clear palimpsest texture, which significantly transform the host rock. The spots and thermally metamorphosed minerals are developed, and the color of the host rock also changes significantly, such as the discoloration of some argillaceous host rocks in Well NP280 after baking metamorphism, from dark gray to yellowish or grayish white; (2) The relatively low-grade metamorphic mudstone or sandstone has palimpsest texture, which transform the host rock weakly, mainly based on the texture and color of the host rock. The spots and thermally metamorphosed minerals are less developed [26]. Therefore, the host rock of the overlying strata of No. 1 intrusion should be sedimentary rocks, which are transformed into medium- to low-grade thermally metamorphic rocks by thermal baking of the intrusion.
Based on the above research, the distribution of igneous intrusion and reservoir in contact metamorphic aureole was accurately predicted by 3D seismic. No. 1 intrusion covers an area of about 25 km2. The top structure is high in south and low in north, and tilts towards northeast, and there are two high points in south and west, respectively (Fig. 8a ). The thickness near the fault-magma conduit is relatively large, and it gradually thins from west to east, the thickness in the central part at about 70-80 m, and gradually thins and bifurcates towards the edge. The area of the contact metamorphic aureoles is slightly smaller than the intrusion, about 20 km2 and 30-56 m thick, with a variation trend basically consistent with that of the intrusion (Fig. 8b ).
Fig. 8. Distribution of No. 1 intrusion and reservoirs in contact metamorphic aureole. |
3. Reservoirs in the contact metamorphic system
3.1. Reservoir physical properties
In the intrusion contact metamorphic system, the metamorphic rocks of the contact metamorphic aureole are dominant reservoir rocks. The reservoir space of the intrusive diabase is undeveloped. The average core porosity of diabase is 5.1% in the Langgu sag and 3.5% in the Nanpu sag [4]. Both are relatively tight and can serve as barriers for the reservoirs of the metamorphic aureole. The metamorphic rocks of the contact metamorphic aureole have good physical properties, and have led to oil and gas discoveries in multiple areas of the Bohai Bay Basin, including the Caojiawu area in the Langgu sag, where the average porosity of the spotted slate is 18.6% with the maximum is 23.3%, and the reservoir space is primarily composed of matrix pores [4].
In the upper metamorphic aureole of No. 1 intrusion, the average porosity of the metasandstone is over 11.5%, and that of the metamudstone is 6.8%. The metamudstone exhibits reservoir characteristics, and cannot serve as a barrier, so the metasandstone and the metamudstone are deemed to be a set of reservoir unit. Affected by different lithology of the host rock, the internal metamorphic aureole consists of interbedded high- quality and common reservoirs. Compared to sedimentary rocks of the same kind at similar depth and original sedimentary environment, the average porosity of the non-metamorphic siltstone is 6.8%, and the maximum is 11.3%. The average porosity of the non-metamorphic mudstone is 4.1%, and the maximum is 4.8%. After metamorphism, the average porosity of the metamorphic siltstone is over 11.5% with the maximum is 18.4%, that of the metamorphic mudstone is 7.5% with the maximum is 13.1%. It is evident that the physical properties of metamorphic siltstone and mudstone are significantly better than those of sedimentary rocks at the same depth (Table 1 ). The high-quality reservoirs are mainly metamorphic fine sandstone, metamorphic siltstone and spotted slate, with generally good oil-bearing properties. The reservoir space is primarily composed of matrix pores, the porosity ranges from 11.5% to 13.8% and the permeability is (10-25)×10−3 μm2. The reservoir is conventional low-porosity and low-permeability reservoir (Fig. 9 ).
Table 1. Physical properties of metamorphic rocks in the upper and lower metamorphic aureoles of No. 1 intrusion and sedimentary rocks not affected by the intrusion |
| Type | Lithology | Well | Formation | Depth/m | Porosity of siltstone/% | Porosity of mudstone/% | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| Maximum | Minimum | Average | Maximum | Minimum | Average | |||||
| Metamorphic | Metamorphic rock | NP280 | Sha3 Member | 3 743.60-3 745.60 | 14.2 | 9.9 | 11.9 (4) | |||
| NP280-41 | Sha3 Member | 4 170.00-4 190.50 | 13.1 | 8.9 | 11.5 (11) | 12.7 | 4.3 | 6.8 (8) | ||
| 4 266.50-4 291.70 | 18.4 | 5.6 | 13.8 (14) | 13.1 | 5.1 | 7.5 (10) | ||||
| Non- metamorphic | Siltstone and mudstone | NP128 | Dong3 Member | 3 726.20-3 760.50 | 8.2 | 5.2 | 6.7 (2) | |||
| NP306x1 | Sha1 Member | 4 237.21-4 239.17 | 4.8 | 3.7 | 4.1 (5) | |||||
| PuGu2 | Sha1 Member | 4 248.61-4 250.00 | 11.3 | 1.7 | 6.8 (3) | |||||
Note: the value in parentheses means the number of samples. |
Fig. 9. Logging interpretation of physical properties and oil-gas-bearing characteristics of reservoirs in the upper and lower metamorphic aureoles of No. 1 intrusion (note that only the top and bottom of the intrusion are shown). |
3.2. Main types of reservoir space
In metasandstone, the primary reservoir space is matrix pores modified by intergranular metamorphic minerals. In metamudstone, the reservoir space primarily consists of fractures accompanied by dissolution vugs. Pores in metasandstone caused by thermal baking appeared during the early diagenetic stage. When the burial depth reaches the late diagenetic stage at 4 176 m, most of quartz and feldspar particles are in linear contact, and basically maintain their original characteristics, only leaving some feldspar particles undergoing alteration. Small particles of intergranular matrix are re-crystallized into thermally metamorphic minerals, around which most pores are transformed into "intergranular pores". The porosity is generally 11.5%, and the permeability is 3×10−3 μm2 (Fig. 10a ). The other type is the matrix pores in spotted slate whose host rock is fine silty mudstone. Most small clastic particles are smaller than 0.01 mm and are distributed in layers, and locally reaching 10% to 30% in content. They are reformed and crystallized into new thermal metamorphic minerals under the action of thermal baking metamorphism, and the pores are mainly secondary "intergranular pores" distributed along the thermally metamorphic minerals in sandy grain layers. The porosity reaches 12.4%, which is significantly larger than that of the host rock at the same depth (Fig. 10b ). In addition, some metasandstone shows obvious hydrothermal dissolution, and the dissolved pores (vugs) are mainly distributed near the intrusive dikes close to the contact face, with approximately 3-5 mm diameter (Fig. 10c ). These matrix pores exhibit strong heterogeneity due to the influence of the host rock and metamorphic minerals. That is, pores are not developed in the thermally metamorphic minerals which are layered and dense, but they are more developed among the thermally metamorphic minerals which are layered, resulting in vertical heterogeneity (Fig. 10d ). On a planar scale, thermally metamorphic minerals are distributed as spots, resulting in planar heterogeneity (Fig. 10e ).
Fig. 10. Matrix pores, fractures, and dissolved pores and oil-bearing features in the upper metamorphic aureole of No. 1 intrusion. (a) Intergranular pores whose original pore structure has been modified by epigenetic thermal metamorphic minerals in the metamorphic fine sandstone in the upper metamorphic aureole, single polarized light (100X), Well NP280-41, 4 176.50 m; (b) pores in the spotted slate in the upper metamorphic aureole distributed around the metamorphic minerals, cast thin section + single polarized light (100X), Well NP203-50, 4 168.58 m; (c) dissolved pores (red arrow) in the metamudstone (whose original rock is dark gray and became ochre after baked), with small vitreous dikes (green arrow), core photo, Well NP280, 3 853.10 m; (d) layered oil-bearing pores in the metamorphic siltstone matrix and bright blue oil-bearing fluorescence, fluorescence and polarized light overlaying on thin section, Well NP203-50, 4 168.58 m; (e) matrix pores with spotted oil in sandy slate, and bright blue hydrocarbon distributed around dark metamorphic minerals, fluorescence and polarized light overlaying on thin section, Well NP203-50, 4 166.94 m; (f) fractures containing oil and oil contamination from fractures to matrix, and blue oil fluorescence, fluorescence and polarized light overlaying on thin section, Well NP203-50, 4 173.25 m. Qtz—quartz. |
There are three types of fractures developed in metamudstone: large structural fractures, small compression fractures, and cooling-induced shrinkage micro-fractures [28]. Large structural fractures are generally high-angle fractures, with 30° scratches on the fracture surface, reflecting the relative horizontal motion of the high-angle fractures. The fracture surface is hardly filled, but obviously dissolved, and with calcite crystals and brown crude oil contamination. Extensive core and imaging logging data show that large structural fractures in the contact metamorphic aureole of No. 1 intrusion are undeveloped. Small compression fractures are caused by the strong compressive stress when magma enters the host rock. These fractures are dipped at 13° to 45° toward northwest, and oriented nearly east-west. Especially in the metamudstone of xenoliths, clear compression fractures can be seen, which are not filled and contain oil and gas (Fig. 4 ). In thin sections, fractures along layers and "X"-shaped conjugate fractures are found, but most of them are filled with calcite, and those not filled generally contain oil and gas (Fig. 10f ). Cooling-induced shrinkage micro-fractures are the result of rapid cooling of magma in the metamorphic aureole near the contact surface. They are "worm-like" and irregularly distributed and nearly perpendicular to the layers, and 2-3 mm long. Some of them have been filled with calcite.
3.3. Distribution of reservoir space
The reservoir lithology of the contact metamorphic aureole is complex, and the reservoir space is highly heterogeneous. It is difficult to clearly reflect the distribution characteristics of the reservoir space by using the available casting thin section technology, so we describe the hydrocarbon distribution in the reservoir to indirectly characterize the distribution and connectivity of the reservoir. Previous polarized and fluorescent thin-section methods failed to represent both the complex pore space and the oil-bearing properties adequately in the same dimension, making it challenging to meet the requirements for characterizing the microscopic reservoir space in the contact metamorphic aureole. This study applies a hybrid technology of incandescent and fluorescent light sources to superimpose the fluorescent display on the single polarized thin section that reflects the texture, visually showing the oil-bearing characteristics in the complex pore structure, to identify and characterize the microscopic reservoir space. The results indirectly reflect three distribution characteristics of metamorphic reservoirs: (1) The layered oil-bearing property reflects that the reservoir space is developed along beddings, and oil distributed in layers in the pores around metamorphic minerals (Fig. 10d ); (2) The spotted oil-bearing property reflects that the reservoir space is developed irregularly, and pores are not developed and the fluorescence is weak where metamorphic minerals are dense, while pores are distributed in patches and the fluorescence is strong where metamorphic minerals are less developed (Fig. 10e ); (3) Oil impregnation along fracture reflects that the reservoir space is developed in stripes, and oil occupies unfilled fractures, and contaminates from the fractures to the matrix on both sides of the fractures (Fig. 10f ).
4. Main controlling factors and exploration potential of reservoirs in contact metamorphic aureole
The formation and distribution of reservoirs in contact metamorphic aureoles are mainly controlled by both external and internal factors. The external factor mainly refers to the intensity of magma baking, and the influencing factors mainly include the thickness of intrusions, the distance from intrusions, and the distance from conduits. Internal factors mainly refer to the sedimentary facies and diagenetic stage of the surrounding rock during magma intrusion. Within the same target layer, the intensity of thermal baking has a greater impact on the development of reservoirs in contact metamorphic aureoles, that is, the greater the intensity of thermal baking, the thicker the reservoirs in contact metamorphic aureoles. In an intrusion contact metamorphic system, lithologic traps formed by the combination of metamorphic reservoirs controlled by a single intrusion and closed tight surrounding rock are favorable targets for deep near-source oil and gas exploration.
4.1. Thermal baking intensity controls the distribution of reservoirs in contact metamorphic aureole
The influence of the intensity of thermal baking on the intrusion is in turn determined by the thickness of the intrusion, the distance from intrusions, and the distance from the magma conduit. Statistical analysis of intrusions and reservoirs of contact metamorphic aureoles in the Bohai Bay Basin and adjacent areas indicates that the reservoir space distribution in the contact metamorphic aureole is mainly controlled by the thickness of the intrusion, with the distribution range slightly smaller than the intrusion. The reservoir thickness is positively correlated with the thickness of the intrusion, approximately 1/3 to 1/2 of the latter (Fig. 11a ). The two metamorphic aureoles are developed at the top and bottom of the same intrusion, and the upper one is slightly thicker than the lower one, with the reservoir more developed than the lower one (Fig. 8a-8b , Fig. 12 ). When the single-layer of intrusion is thicker than 30 m, strong baking develops thick metamorphic aureoles with clear electrical characteristics and significant reconstruction, making them the most noteworthy targets for oil and gas exploration (Fig. 3 ). In contrast, when the single-layer of intrusion is 10-30 m, baking and metamorphic effects become weak, resulting in atypical characteristics of metamorphic aureoles, which have no obvious effects on reservoir reconstruction. When the single-layer of intrusion is less than 10 m, weak baking and metamorphism make metamorphic aureoles undeveloped [5,28]. In addition, in the interval where intrusions are densely developed with thick interbeds, the more interbeds there are, the more intensive the thermal baking, the thicker the metamorphic reservoirs, and the more developed the high-quality reservoirs.
Fig. 11. Control of intrusion thickness and distance from magma conduit on reservoir development in contact metamorphic aureoles. |
Fig. 12. Influences of the thickness of No. 1 intrusion and the distance from intrusions on reservoir distribution in contact metamorphic aureoles (① Fracturing stimulation: open flow, 5 mm oil nozzle, oil at 36.3 t/d, and gas at 4.0×104 m3/d; ②Fracturing stimulation: electric pump, 10 mm oil nozzle, oil at 13.9 m3/d, and gas at 6 320 m3/d; ③ Fracturing stimulation: open flow, liquid production at 15.4 m3/d (including oil at 0.3 m3/d); ④ Fracturing stimulation: 5 mm oil nozzle, oil production at 8.37 m3/d and gas at 151 m3/d; ⑤ Fracturing stimulation: nitrogen displacement, liquid production at 240 m3/d, profile position in |
The distance from intrusions varies, and the surrounding rock is subjected to different levels of thermal baking, mechanical extrusion and hydrothermal dissolution, resulting in different thicknesses of reservoirs in metamorphic aureole. The closer it is to the intrusion, the stronger the thermal baking effect on the surrounding rock, the higher the degree of contact metamorphism, and the greater the brittleness of the rock, generally transitioning from gray-white slate to gray-dark gray host rock. At the same time, the compressive stress is also greater, the compression fractures are more developed, and the reservoirs in metamorphic aureole are also more developed (Figs. 9 and 12 ).
When magma leaves the conduit and spreads to the outer edge of the main area, the baking intensity weakens as the magma temperature decreases, and the thickness of the metamorphic aureole gradually decreases. The closer to the conduit, the higher the magma temperature, the greater the baking intensity is, and the greater the thickness of the reservoir in metamorphic aureole. In the middle slightly away from the conduit, as the magma temperature gradually decreases, the baking intensity weakens, and the thickness of the reservoir in metamorphic aureole is smaller. In the far edge away from the conduit, the thickness of the intrusion is small, the magma temperature is low, and the baking intensity is weak, resulting in a thin or undeveloped reservoir in metamorphic aureole (Fig. 11b ).
4.2. Coupling of favorable lithofacies and thermal contact metamorphism, intrusive compression and hydrothermal dissolution controls the development of high-quality reservoirs
In the intrusion contact metamorphic system, the formation of contact metamorphic aureole reservoirs is mainly affected by three factors.
(1) The reconstruction of favorable lithofacies pores by thermal contact metamorphism is essential for the formation of high-quality reservoirs. In the metamorphic alteration of intergranular pores in deep tight host rocks, favorable lithofacies control the storage capacity of reservoir in the contact metamorphic aureole. In pro-deltas, siltstone and silty mudstone are the best, which can be reconstructed from deep unconventional reservoirs into "conventional reservoirs". When suffered from thermal baking metamorphism at high temperature and normal pressure (normal pressure gradient), fine particles smaller than 0.01 mm in siltstone and silty mudstone are more likely to undergo recrystallization and develop into new thermally metamorphic minerals or metamorphic mineral stripes, around which new secondary "intergranular pores" may be produced, and thus developing metamorphic siltstone-silty mudstone reservoirs (Fig. 10b ). Particularly, when magmatic intrusion happened in the early diagenetic stage, such modification to the original intergranular pore structure will be "preserved", and prevent from being affected by later diagenetic compaction.
(2) The intrusive compression increases the fractures related to intrusion. Magma intrusion can induce compressive fractures and cooling-induced shrinkage fractures in the contact metamorphic aureole, thereby increasing the reservoir space. As magma intrudes into the formation, it exerts a compressive-shear stress on the host rock in the same direction as the magma flow, resulting in a series of small-scale low-angle compressive fractures, including fractures in xenoliths, various bedding fractures in the metamorphic aureole, and "X-shape" conjugate fractures (Figs. 4 and 10f ). For example, a group of fractures with low angles of 13°-45° were found in the imaging logging data of the lower metamorphic aureole in Well NP2-66, which was caused by the extrusion of magma flow on the host rock. Compressive fractures provide effective pathways to enhance connectivity and communication among matrix pores. Cooling-induced shrinkage microfractures in the metamorphic aureole are a result of uneven cooling, and they are almost filled with calcite.
(3) Hydrothermal dissolution leads to the formation of dissolved vugs, thereby increasing reservoir space. The CO2-rich hydrothermal fluids carried by basic magma and the organic acids formed by thermal evolution of organic matter along the vicinity of intrusive dikes dissolve the easily soluble minerals such as feldspar and calcite in the contact metamorphic aureole, forming dissolution vugs that can enhance the reservoir space. For example, dissolution is quite common in Well NP280 near the intrusion conduits (Fig. 10c ).
4.3. The lithologic trap near the contact metamorphic aureole is the favorable target for deep exploration.
The introduction of the intrusion contact metamorphic system has improved the understanding of the genesis and distribution of deep igneous reservoirs, changed previous fragmented understanding of eruptive andesite, rhyolite, and volcaniclastic rock obtained from studying reservoir segments separately, and helped to study the development mechanism and distribution law of favorable reservoirs. On the other hand, it provides a new target reservoir for deep near-source oil and gas exploration. In an intrusion contact metamorphic system, the reservoirs in the metamorphic aureole are controlled by the intrusion and the tight host rock which can form a perfect lithologic trap, that is, a lithologic trap with the intrusive rock as the top or the bottom, the metamorphic aureole as the reservoir, and the tight host rock as the cap rock or barrier bed. Such lithologic traps are mostly located in the forefront of delta and pro-delta facies. Sandstone and sandy mudstone become metamorphic reservoirs and favorable targets for deep near-source oil and gas exploration after metamorphism. Taking the deep layers in the Nanpu sag as an example, 27 metamorphic lithologic traps have been discovered in the Dong3 Member to the Sha3 Member, with a superimposed area of 796 km2. The total reservoir thickness in the upper and lower metamorphic aureoles is over 40 m, and the predicted trap resources are over 1×108 t (Fig. 1 ). The Laoyemiao intrusion in the lower part of the Dong3 Member developed with metamorphic aureoles covered 145 km2, which built a lower-source and upper-reservoir model together with oil source faults. Significant oil and gas discoveries have been made there. It is evident that intrusion contact metamorphic targets have become a new target for deep near-source oil and gas exploration in the Nanpu sag. The proposal of intrusion contact metamorphic system will promote the investigation and exploration discovery in deep metamorphic reservoirs in other hydrocarbon-rich sags (i.e., Qikou sag, Langgu sag, northern part of the Eastern Liaohe sag).
5. Conclusions
The intrusion contact metamorphic system is a macroscopic reservoir model based on the inherent genetic and spatial symbiotic relationship between the intrusion and contact metamorphic aureoles. The model has five structural layers of the intrusion, upper metamorphic aureole, lower metamorphic aureole, normal sedimentary layers on the roof and the floor; and there are three basic elements of intrusion, contact metamorphic aureoles (including metamorphic reservoirs) and normal sedimentary layers.
It is believed that No. 1 igneous body is of intrusive origin based on intrusion contact metamorphic system model, and its five vertical structural layers are consistent with other regions in the Bohai Bay Basin. There are xenoliths at the top of the intrusion with indicating meaning, the microscopic rock texture and mineral crystallization systematically change from the center to the edge of the intrusion, and macroscopically, the intrusion penetrates the isochronous framework at a low angle. The host rock of the metamorphic aureole is sedimentary rock with typical palimpsest textures and various thermally metamorphic minerals and lithologies changed at low to middle temperature. The reservoir space of the metasandstone is dominated by matrix pores modified by intergranular metamorphic minerals, and that of the metamudstone is mainly characterized by fractures accompanied by dissolution vugs. The complex reservoir space has three distribution characteristics: layered, porphyritic and hydrocarbon impregnation along fracture.
The formation and distribution of reservoirs in contact metamorphic aureoles are mainly controlled by both external and internal factors. The intensity of thermal baking controls the spatial distribution of the reservoirs in the contact metamorphic aureole. The coupling of favorable lithofacies and thermal contact metamorphism, intrusive compression and hydrothermal dissolution controls the development of high-quality reservoirs. The introduction of the intrusion contact metamorphic system has expanded the research ideas and methods of reservoirs in contact metamorphic, and it is also a significant reference for the study and discovery of exploration targets of deep contact metamorphic rock in the Nanpu sag and other oil-gas-rich sags.
Acknowledgement
We would like to express our sincere gratitude to Professor Dong Yuexia of CNPC Advisory Center and experts from Jidong Oilfield Project Team for their strong support and assistance in the project research, especially to engineers Wang Shuqin and Yang Jinying for their help in the preparation of core slices.