Guanghui Yuan

Characteristics and origin of abnormally high porosity zones in buried Paleogene clastic reservoirs in the Shengtuo area, Dongying Sag, East China

There are three abnormally high porosity zones developed in buried Paleogene nearshore subaqueous fan and sublacustrine fan clastic reservoirs at 2,800–3,200 m, 3,250–3,700 m and 3,900–4,400 m, respectively, within the Shengtuo area of the Dongying Sag. Here the porosity of reservoirs buried deeper than 4,000 m can still be greater than 20%. Investigation of these three abnormally high porosity (AHP) zones in the 3rd to 4th member of the Paleogene Shahejie Formation in the Shengtuo area was carried out with utilization of core observation, thin section identification, SEM observation, image analysis, core physical property testing and other technical methods. The results show that, the AHP zones in 2,800–3,200 m and 3,250–3,700 m are visible pores primary AHP zones dominated by significant primary intergranular pores (more than 50% of the total porosity), while secondary pores and micropores in authigenic clays may develop in some reservoirs. AHP reservoirs in the AHP zone of 3,900–4,400 m are dominated by micropores in matrix, visible pores are mainly grain dissolution pores but with low absolute content (< 1%), so this zone belongs to the micropores primary AHP zone. The genesis of the three AHP zones was studied to distinguish between porosity enhancement and porosity preservation. Our research shows that, in deeply buried clastic reservoirs in the Shengtuo area, mineral dissolution occurred in a relatively closed diagenetic system with high temperature and high salinity. Reservoir rocks underwent extensive feldspar dissolution, while detrital carbonate grains and carbonate cements show no evidence of extensive dissolution. Although significant feldspar dissolution pores developed, feldspar dissolution enhanced porosity only a little due to the precipitation of almost isovolumetric dissolution products in the nearby primary intergranular pores in forms of authigenic clays and quartz cements. Net enhanced porosity originating from feldspar dissolution is generally less than 0.25%. Thus, the subsurface dissolution has little impact on the mid-deep buried high porosity reservoirs. Reservoirs in braided channels of middle fans in sublacustrine fans and reservoirs in the middle-front of fan bodies of nearshore subaqueous fans provide the basis for the development of AHP zones. The shallow development of fluid overpressure and early hydrocarbon emplacement have effectively retarded compaction and carbonate cementation, so that the high porosity in the superficial layers is preserved in the mid-deep layers. These are the main controlling factors in the development of AHP zones.

Feldspar dissolution, authigenic clays, and quartz cements in open and closed sandstone geochemical systems during diagenesis: Typical examples from two sags in Bohai Bay Basin, East China

Feldspar dissolution and precipitation of clays and quartz cements are important diagenetic reactions affecting reservoir quality evolution in sandstones with detrital feldspars. We examined two sets of sandstone reservoirs to determine whether the sandstone diagenetic systems were open or closed to the mass transfer of products from feldspar dissolution and its impact on reservoir quality. One of the reservoirs is the Eocene fan delta sandstone buried 2.5–4.0 km (1.5–2.5 mi) below sea level (BSL) in the Gaoliu (GL) area of the Nanpu sag, and the other is the Eocene subaqueous fan sandstone buried 1.5–4.5 km (1–2.8 mi) BSL in the Shengtuo (ST) area of the Dongying sag. Both sandstones consist mainly of lithic arkoses and feldspathic litharenites, and have secondary porosity formed by dissolution of feldspars. In the GL sandstones, the absolute amounts of authigenic clays and quartz cements (generally 125°C [257°F]). The low abundance of authigenic clays and quartz cements, and low pore-water salinity indicate that much of the , , and released from leached K-feldspars were exported from the GL sandstone system. And the extensive feldspar dissolution enhanced much porosity and permeability. In contrast, the ST sandstones with secondary pores formed by feldspar dissolution generally contain authigenic clays (kaolinite and illite) and quartz cements with almost identical volume of secondary pores. Kaolinite dominates in the ST sandstones at shallower depth (3.1 km [2 mi] BSL) where temperature exceeds 125°C (257°F). The presence of abundant clays and quartz cements indicates that and released from leached feldspars were retained in the ST sandstone system. The dominance of authigenic illite at greater depth indicates that sufficient should have been retained within the sandstones for occurrence of illitization of kaolinite and feldspars. Secondary porosity in thin sections can be up to 3%, but little porosity ( The diagenetic difference between the GL and the ST sandstones can be interpreted by assessing pore-water evolution in these two areas. The current pore waters with low salinity and negative hydrogen isotopic compositions in the GL sandstone system indicate the significant impact of meteoric water, whereas the current pore waters with high salinity and the paleofluids with positive oxygen isotopic compositions in the ST sandstone system indicate little trace of meteoric water. Access of meteoric freshwater to the GL area probably occurred during the late Oligocene to Neogene through widely developed faults in the Paleogene and Neogene strata. The low-salinity water could have been responsible for flushing of solutes derived from feldspar dissolution. As such, diagenesis in the GL sandstones is considered to have occurred in an open geochemical system, whereas with limited faults and high water salinity, the ST sandstones acted as a closed geochemical system where precipitation of kaolinite, illite, and quartz cements occurred following dissolution of feldspars.

Characteristics and formation mechanisms of large volcanic rock oil reservoirs: A case study of the Carboniferous rocks in the Kebai fault zone of Junggar Basin, China

Volcanic hydrocarbon reservoirs are rare and may be overlooked. The Carboniferous volcanic rocks of the Kebai fault zone in the western Junggar Basin contain hydrocarbon (HC) reservoirs in volcanic rock with proven oil reserves of 9.76 × 108 bbl that have a complex filling history. We have investigated the lithology and properties of these volcanic rock HC reservoirs as well as diagenesis and control of faults and fractures in oil reservoirs. The lithology of these Carboniferous volcanic rocks is primarily andesite and tuff. Also present were volcanic breccia and metamorphic rock in addition to rhyolite, felsite, diabase, and granite in the volcanic lava. On the basis of microscopic examination, five types of pores and fractures were observed: (1) fracture–dissolved phenocrystal pore, (2) fracture–intergranular pore, (3) fracture–gas pore, (4) fracture–dissolved intragranular pore, and (5) fracture–dissolved matrix pore. The fractures in these rocks are a significant factor in connecting the pores. Diagenetic processes that control reservoir quality include compaction, filling of pores and fractures, cementation, metasomatism, and grain dissolution. The volcanic reservoirs show a variety of lithologies, and oil has been discovered in all types of Carboniferous rocks. The controlling factors for oil distribution in these Carboniferous volcanic rocks are faulting, fracture development, and degree of weathering when they were subaerially exposed in the Permian. The area in which these faults and fractures developed is the primary area of oil enrichment with high yields. The objectives of this study were to (1) describe the characteristics of different types of volcanic rocks and reservoirs found in this basin and (2) characterize the diagenetic history of these rocks and document how diagenesis controls porosity and permeability.

Diagenesis impact on a deeply buried sandstone reservoir (Es1 Member) of the Shahejie Formation, Nanpu Sag, Bohai Bay Basin, East China

Abstract Diagenesis has a significant impact on reservoir quality in deeply buried formations. Sandstone units of the Shahejie Formation (Es1 Member) of Nanpu Sag, Bohai Bay Basin, East China is a typical deeply buried sandstone with large hydrocarbon accumulations. The methodology includes core observations and thin section studies, using fluorescence, scanning electron microscope (SEM), cathodoluminescence (CL), fluid inclusion and isotope and electron probing analysis as well as the numerical determination of reservoir characteristics. The sandstones consist of medium to coarse-grained, slight to moderate sorted lithic arkose and feldspathic litharenite. Porosity and permeability values range from 0.5 to 30% and 0.006 to 7000 mD, respectively. The diagenetic history reveals mixed episodes of diagenesis and deep burial followed by uplift. The main diagenetic events include compaction, cementation alteration, dissolution of unstable minerals and grain fracturing. Compaction resulted in densification and significantly reduced the primary porosity. Quartz, calcite and clay are the dominant pore-occluding cement and occur as euhedral to subhedral crystals. Alteration and dissolution of volcanic lithic fragments and pressure solution of feldspar grains were the key sources of quartz cement whereas carbonate cement is derived from an external source. Clay minerals resulted from the alteration of feldspar and volcanic lithic fragments. Porosity and permeability data predict a good inverse relationship with cementation whereas leaching of metastable grains, dissolution of cement and in some places formation of pore-lining chlorite enhanced the reservoir quality. The best reservoir is thicker sandstone bodies that are medium to coarse-grained, well-sorted sandstone with low primary ductile grains with a minor amount of calcite cement. The present study shows several diagenetic stages in the Es1 Member, but the overall reservoir quality is preserved.

Petrography, fluid-inclusion, isotope, and trace-element constraints on the origin of quartz cementation and feldspar dissolution and the associated fluid evolution in arkosic sandstones

Feldspar dissolution, quartz cementation, and clay cementation are significant diagenetic processes in deep-water fan feldspar-rich sandstones in the Shengtuo area, Dongying sag (East China). The timing and origin of these water–rock interactions as well as the paleofluids in which relevant chemical reactions occurred were deduced using data from microscopy, fluid-inclusion microthermometry, oxygen-isotope microanalysis, and trace-element microanalysis. Three distinct, separate episodes of quartz overgrowths (referred to as Q1, Q2, and Q3) were distinguished by cathodoluminescence microscopy. The Q1 quartz, identified in all porous sandstones from 2500 to 3600 m (8200 to 11,800 ft), was formed at approximately 100°C–115°C (212°F–239°F) before oil filled the reservoirs. The Q2 quartz was mainly precipitated at 115°C–130°C (239°F–275°F), accompanying or slightly postdating the main phase of oil filling, and was identified in samples from 2800 to 3600 m (9200 to 11,800 ft). The Q3 quartz was only identified in sandstones buried deeper than 3500 m (11,400 ft) and was likely precipitated in the Quaternary when temperature exceeded 130°C–135°C (266°F–275°F). Secondary ion mass spectrometry oxygen-isotope microanalyses yielded δ18OVSMOW (Vienna standard mean ocean water) values ranging from 21.42‰ to 24.35‰ for Q1 quartz, from 22.03‰ to 24.99‰ for Q2 quartz, and from 21.72‰ to 22.91‰ for Q3 quartz. A mass-balance calculation and quantitative petrography data of the amount of leached feldspars and associated secondary minerals suggest that the internal feldspar dissolution is likely the primary source for the authigenic clays and quartz cements in these sandstones. Positive δ18O(water) values (+0.5‰ to +4.5‰ VSMOW) of quartz-forming waters indicate that quartz cementation and feldspar dissolution occurred in a geochemical system with a limited volume of diagenetically modified connate water. The variations of δ18O(cements) and trace-element compositions from Q1 quartz to Q2 quartz in individual overgrowth suggest that hydrocarbon filling changed the chemistry of the pore fluid significantly; further, δ18O(water) values of the pore water increased by approximately 2‰ VSMOW after oil filling. Meteoric freshwater did not contribute to quartz cementation and simultaneous feldspar dissolution. The detected CO2 and hydrocarbons in fluid inclusions in the quartz cements, the existence of CO2 in hydrocarbon-rich natural gas, and the pyrobitumen in the feldspar-hosted pores suggest that acids derived from organic matter (kerogen in source rocks and hydrocarbons in reservoirs) probably have promoted the extensive subsurface leaching reactions of feldspars in these sandstones. The ongoing development of Q2 and Q3 quartz suggests that quartz precipitation did not cease after oil filling. Further, diagenetic reactions likely have proceeded from water–rock interactions to hydrocarbon–water–rock interactions.

Genetic mechanism of high-quality reservoirs in Permian tight fan delta conglomerates at the northwestern margin of the Junggar Basin, northwestern China

Fan delta conglomerate reservoirs in the Permian Jiamuhe Formation in the Zhongguai area at the northwestern margin of the Junggar Basin, northwestern China, are reservoirs for large accumulations of natural tight gas. The tight conglomerates and sandstones are mainly litharenites with a large amount of texturally and compositionally immature volcanic clastic materials. Core porosities demonstrate the development of anomalously high porosities at depths of 3200–4200 m (10,500–13,800 ft) and 4500–4900 m (14,800–16,100 ft). The Permian reservoirs experienced initial rapid subsidence, uplift, and further subsidence, and diagenetic reactions occurred in these reservoirs involving compaction; precipitation of chlorite clays, calcites, zeolites, iron oxides, kaolinite, and quartz cements; and dissolution of unstable minerals including analcite, laumontite, feldspars, and rock fragments. Low-porosity reservoirs, with extensive compaction and (or) cementation of calcite and heulandite-Ca, consist of only few visible secondary pores formed by dissolution of feldspars and rock fragments. Reservoirs with anomalously high porosity, however, experienced relatively weak compaction and contain significant amounts of secondary pores formed by dissolution of mainly laumontites, some feldspars, and rock fragments. Comprehensive studies of sedimentary features and diagenesis of the reservoirs indicate that these anomalously high porosities originate from chronological coupling of four important geological processes. (1) Sedimentary facies and detrital compositions controlled distribution of various zeolites in reservoirs, with abundant laumonites developed in reservoirs in the fan delta front subfacies. (2) Early precipitation of the laumontites inhibited compaction during deep burial; however, they provided unstable minerals for secondary porosity development. (3) The Permian reservoirs experienced subaerial exposure erosion during the uplift stage, and meteoric freshwater leached laumontite cements and aluminosilicate grains to form secondary pores in reservoirs beneath the unconformity. (4) Hydrocarbon emplacement at relative shallow depth during a second subsidence period preserved the secondary pores in reservoirs by retarding late carbonate cementation.