The common belief among many petroleum geologists that regions of volcanic and metamorphic rocks are generally to be avoided as potential hydrocarbon reservoirs has greatly slowed the research and exploration efforts on hydrocarbon potential in volcanic and metamorphic rocks. However, many hydrocarbon-bearing basins containing volcanic and metamorphic rocks have been found in convergent margin settings and in rift basins. This article describes the reservoir lithofacies and wire-line logs and elucidates the parameters controlling reservoir-quality evolution of Archean metamorphic and Jurassic volcanic rocks from the Xinglongtai buried hill, western depression of the Liaohe basin, China. Four lithofacies (pyroclastics, lavas, volcaniclastics, and volcaniclastic-epiclastics) have been identified in the Jurassic volcanic reservoir rocks, each having different pore types and variable porosity and permeability values and, thus, different reservoir potentials. Pore types in the volcanic rocks include voids, fractures, fissures, weathering cracks, interstices, and vesicles. The volcanic-rock reservoir evolution is primarily controlled by the burial-thermal diagenesis. Plastic deformation and alteration of the biotite during the eogenetic phase led to the considerable loss of primary pores. Destruction of the primary porosity by compaction was limited by the presence of eogenetic carbonate and zeolite cement formation. Dissolution during the deep-burial mesogenetic phase and during near-surface leaching and erosion in the intervening volcanic eruptions enhanced the permeability and increased reservoir quality. The pore types in the Archean metamorphic reservoir include fractures, dissolution voids, and weathering fissures. Where the Jurassic volcanic rocks or the Paleogene source rocks directly cover the weathered zone, the fissures and fractures have remained open, but where the metamorphic rocks are covered by the Mesozoic mudstones, most fissures are filled with mud and iron oxides. Reservoir quality of the Archean metamorphic and Jurassic volcanic rocks is also partly related to the paleogeomorphology of the area. Rocks in the paleohighs and in adjacent transitional areas have enhanced reservoir properties greater than those in paleolows because of more extensive weathering and the development of vugs and fissures.
The reservoir quality of Jurassic and Triassic fluvial and lacustrine‐deltaic sandstones in the intracratonic Ordos Basin is strongly influenced by depositional facies and various types of diagenetic modifications. The fluvial sandstones have higher average He‐porosity and permeability (14.8% and 12.7 mD, respectively) than the deltaic sandstones (9.8% and 5.8 mD, respectively). In addition to extensive mechanical compaction, eodiagenesis (220–97 Ma; depth < 2000 m; T < 70°C) has resulted in dissolution and kaolinitization of detrital silicates in the Jurassic fluvial sandstones, and in smectite infiltration and minor cementation by calcite and siderite in the Triassic fluvial and deltaic sandstones. Pervasive eogenetic carbonate cementation (> 20 vol.%) occurred in Triassic deltaic siltstones and very fine‐grained sandstones which are closely associated with organic‐rich mudstones. Mesodiagenesis (97–65 Ma), which occurred during rapid subsidence to depths of 3700–4400 m, resulted in the albitization of plagioclase, checmical compaction, the conversion of kaolinite into dickite, and cementation by quartz overgrowths, chlorite, illite, ankerite (δ 13 C VPDB =−2.4‰ to +2.6‰; δ 18 O VPDB =−21.5‰ to −10‰) and calcite (δ 13 C VPDB =−4.7‰ to +3.7‰; δ 18 O VPDB =−21.8‰ to −13.4‰). Oil emplacement (95 Ma) retarded cementation by mesogenetic quartz and carbonate but had little influence on dickite, illite and chlorite formation. Retardation of quartz cementation was also due to the presence of chlorite fringes around detrital quartz grains. Dickitization of eogenetic kaolinite together with the short residence time at maximum burial temperatures (105–124°C) has retarded the albitization of K‐feldspars and illite formation and hence prevented severe permeability destruction. Telodiagenesis, which occurred after uplift (Eocene to end‐Neogene), caused slight dissolution and kaolinitization of feldspars. This study demonstrates that despite complex patterns of diagenetic modifications in the Triassic and Jurassic successions, depositional porosity and permeability are better preserved in fluvial meandering channel sandstones than in deltaic sandstones. These results should be important for modelling of reservoir‐quality distribution and exploration risk evaluation in the basin.
The lithofacies and reservoir properties of Cretaceous volcanic rocks in the Fenghuadian Suite (Huanghua Basin, eastern China) are described in this paper. Three lithofacies have been identified (pyroclastics, lavas and volcaniclastics) and each has different reservoir potential due to differences in porosity and permeability. Pyroclastics and lavas have better reservoir properties than do volcaniclastic rocks. In the pyroclastics, voids and fractures within brecciated rocks together with dissolution voids and fissures result in highly connected porosity (up to 22%). In the lavas, vesicles, dissolution joints and cooling fissures together with weathering cracks and fissures are abundant and have high connectivity in the upper portions of each volcanic cycle, giving rise to excellent reservoir properties. By contrast, volcaniclastic rocks have low porosities and poor reservoir potential. We suggest that porosity development is partly related to palaeo‐geomorphology. Thus, volcanic rocks in palaeo‐highs and in neighbouring transitional areas have enhanced reservoir properties as a result of weathering and the consequent development of vugs and fissures. Chromatographic analyses of crude oils and rock extracts indicate that the Kong‐2 Member of the Eocene Kongdian Formation, which overlies the Fenghuadian Suite in the study area and which also occurs in the nearby Cangdong Depression, is the principal source of hydrocarbons reservoired in the Fenghuadian Suite. Faults may have acted as conduits for hydrocarbon migration.
To clarify the reservoir characteristics of laminated shale, the occurrence mechanism of shale oil and its influencing factors in the Gulong Sag, northern Songliao Basin, are studied to better guide the exploration and development of shale oil there. First, X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) are used to characterize the pore types, pore geneses and factors influencing the pore volume in the study area. Second, the organic matter of the samples is extracted with a mixture of dichloromethane and methanol. Total organic carbon (TOC), nitrogen adsorption and Rock-Eval tests are performed on the samples before and after extraction to reveal the pore size distribution after extraction. The factors influencing free and adsorbed shale oil and the lower limit of pore size are discussed in detail. The results show that interparticle pores (interP pores), intraparticle pores (intraP pores), organic matter pores (OM pores) and microfractures can be found in the laminated shale (Q1) in the Gulong Sag, Songliao Basin, and that the interP pores and intercrystalline pores in clay minerals are the main pores. The FE-SEM results show that the diameters of interP pores vary from several hundred nanometers to several microns, and their morphologies are mainly triangular, strip-shaped or irregular. The morphology of the intercrystalline pores in the clay minerals is generally irregular, depending on the crystal type and arrangement of clay minerals. According to the characteristics of the nitrogen adsorption and desorption curves, the pore morphologies are mainly slit-shaped pores, parallel-plate-shaped pores and ink-bottle-shaped pores. The pore size distribution is mostly bimodal, and the pore volume contribution is the greatest in the pore size range of 10~20 nm. Before and after extraction, the overall characteristics of the pore size distribution change only slightly, but the number of micropores increases significantly. Different minerals have different degrees of influence on the proportions of micropores, mesopores and macropores. Quartz mainly inhibits the formation of micropores, while the overall effect on mesopores and macropores is positive depending on the diagenetic period. Feldspar has a strong positive correlation with the micropore and mesopore proportions but is not highly correlated with the macropore proportions. The influence of the carbonate mineral content on the pore volume is not obvious because of its complex composition. The TOC content and vitrinite reflectance (Ro) are the two most important factors controlling free oil and adsorbed oil, and the contents of mineral components, such as felsic minerals, carbonate minerals and clay minerals, have no obvious correlation with shale oil content. With increasing pore volume, the contents of free oil and adsorbed oil increase, but the proportion of adsorbed oil decreases gradually. The correlation between the specific surface area and adsorbed oil content is poor. At normal temperatures and pressures, the lower limit of the pore diameters that can contain free oil is 4 nm, and the lower limit of the pore diameters that can contain movable oil is 10 nm.
Abstract The granodiorites, monzogranites and diorites are widely developed in the Balikun area of Eastern Tianshan Orogen. Of which, LA–ICP–MS zircon U‐Pb isotopic dating from the diorites revealed that they were emplaced at 327∼333 Ma, representing an important period of magmatism in the Early Carboniferous. Geochemically, they are characterized by moderate SiO 2 (51.33–62.48 wt%), high but variable MgO (2.04–11.16 wt%, average 5.35), higher Mg # (40–73) and TiO 2 (0.67–1.29 wt%), Na 2 O/K 2 O (1.39–2.95) as well as variable Cr (2.49–675 ppm) and Ni (1.31–174 ppm), showing a geochemistry similar to those of high‐Mg diorites or sanukitoids. In addition, they are enriched in the LILE, poor HFSE with an evident negative Nb anomalies and a REE pattern of moderate fractionator between LREE and HREE without or weak negative Eu anomalies. Their ∊ Hf ( t ) are positive (+3.63–+15.65), suggesting a source from the depleted mantle. In addition, they have high TiO 2 and Pb, and large quantity of amphibolite and biotite, indicating that they were most likely derived from the partial melting of depleted mantle metasomatized by the slab‐derived melt under a hydrous condition. Consequently, combined with the contemporary volcanics and granitoids formed in the island arc settings, we proposed that the subduction was continued till Early Carboniferous in the Bogda‐Harlik tectonic belt. After that, wide occurrence of the post‐collisional A‐type granites and mafic‐ultramafic intrusions indicate this tectonic belt entered the post‐collisional environment from Late Carboniferous to Permian.
The characteristics of reservoir heterogeneity of the marine gravity flow tight sandstone from the Miocene Huangliu Formation under abnormally high pressure setting at LD10 area in Yinggehai Basin are studied, and the influencing factors on reservoir heterogeneity are discussed, based on modular formation dynamics test, thin sections, XRD analysis of clay minerals, scanning electron microscopy, measurement of pore throat image, porosity and permeability, and high pressure Hg injection, as well as the stimulation of burial thermal history. The aim is to elucidate characteristics of the heterogeneity and the evolution process of heterogeneity of the reservoir, and predict the favorable reservoirs distribution. (1) The heterogeneity of the reservoir is mainly controlled by the cement heterogeneity, pore throat heterogeneity, quality of the reservoir heterogeneity, and the diagenesis under an abnormally high pressure setting. (2) The differences in pore-throat structure caused by diagenetic evolution affected the intergranular material heterogeneity and the pore throat heterogeneity, and finally controlled the heterogeneity of reservoir quality. (3) Compared with the reservoir under normal pressure, abnormally high pressure restrains strength of the compaction and cementation and enhances the dissolution of the reservoir to some extent, and abnormally high pressure thus weakening the heterogeneity of the reservoir to a certain degree. The favorable reservoirs are mainly distributed in the gravity flow sand body under the strong overpressure zone in the middle and lower part of Huangliu Formation.
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