Quantitative prediction of fractures in shale using the lithology combination index

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. Fractures, which are related to tectonic activity and lithology, have a significant impact on the storage and production of oil and gas in shales. To analyze the impact of lithological factors on fracture development in shales, we selected the shale formation from the Da’anzhai member of the lower Jurassic shales in a weak tectonic deformation zone in the Sichuan Basin. We defined a lithology combination index (LCI), that is, an attribute quantity value of some length artificially defined by exploring the lithology combination. LCI contains information on shale content at a certain depth, the number of layers in a fixed length (lithology window), and the shale content in the lithology window. Fracture porosity is the percentage of pore volume to the apparent volume of the rock. In the experiment, fracture porosity was obtained using 50 samples from six wells, by observing rock slices under a microscope. The relationship between LCI and fracture porosity was analyzed based on machine learning, regression analysis, and weighting methods. The results show that LCI is able to represent the impact of multiple lithological factors (i.e., shale content at a certain depth, the number of layers in the lithology window, and the shale content in the lithology window). The LCI within a thickness of 2 m for the lithology window demonstrates a good linear relationship with fracture porosity. We therefore suggest LCI be used for fracture predictions of shale formations from weak tectonic deformation zones. Our proposed LCI and fracture prediction methods also provide implications for sandstone, mudstone, or carbonate formations under similar processes

Sulfate Sources of Thermal Sulfate Reduction (TSR) in the Permian Changxing and Triassic Feixianguan Formations, Northeastern Sichuan Basin, China

Thermal sulfate reduction (TSR) occurred throughout the Permian Changxing (P2c) and Triassic Feixianguan (T1f) dolostone reservoirs in the western and eastern parts of the Kaijiang-Liangping (K-L) trough in the northeastern part of the Sichuan Basin. To determine the sulfate sources of this TSR, fourteen solid bitumen samples and eight anhydrite samples were collected from the northeastern part of the Sichuan Basin. These samples were analyzed to determine their sulfur isotopes. In addition, untreated, HNO3-treated, and CrCl2-treated solid bitumen samples were analyzed to determine their sulfur isotopes in order to obtain reliable δ34S data for the TSR solid bitumen. The results show that the HNO3 method is more effective at removing pyrite from solid bitumen than the method using CrCl2 thrice because the HNO3-treated solid bitumen has lower sulfur contents and higher δ34S. The δ34S of the T1f solid bitumen samples from the Puguang gas field (in the eastern part of the K-L trough, 12.0-24.0‰) is significantly lower than that of the samples from the Yuanba gas field (in the western part of the K-L trough, 24.1-34.2‰). The δ34S of the T1f1–2 anhydrite is 18.1-26.6‰, which is lower than that of the T1f3–4 anhydrite samples (29.9-39.6‰). The TSR sulfates from the Puguang gas field were most likely from the coeval T1f1–2 evaporating seawater and were enriched during the reflux-seepage dolomitization process. The TSR sulfates from the Yuanba gas field were primarily caused by the evaporation of seawater during the T1f4. First, the evaporating seawater would flow vertically into the P2c reservoirs in the adjacent area, and then, it would flow laterally into the P2c reservoirs in the Yuanba gas field. Considering the fact that the sulfate sources of TSR and the δ34S values of the TSR sulfates are different in the Puguang and Yuanba gas fields, the δ34S of TSR solid bitumen cannot be simply used to show the extent of TSR

FABRIC-SPECIFIC ISOTOPIC VARIABILITY IN LATE EDIACARAN DOLOMITES, SOUTH CHINA: ELUCIDATED BY MAGNESIUM, SULPHUR AND CLUMPED ISOTOPES

Dolomite (CaMg(CO (sub 3) )2) as a major carbonate mineral commonly distributes on the shallow-water platform throughout Earth history. Due to its thermodynamic stability, stoichiometric dolomite is less susceptible to the chemical alterations and often regarded as a reliable archive to reconstruct past environmental and ocean chemistry conditions. This is particularly important during the Precambrian time as it lacks of biological proxies. However, using dolomite as a geochemical proxy is challenging due to potential exposure to various diagenetic realms, such as burial or subaerial condition, which can partly alter and, in some cases, completely overprint the original signatures. Such phenomenon will generate large isotopic variations depending on the different dolomite fabrics. To examine the fidelity of geochemical records in dolomite and understand the governing processes of fabric-specific isotopic variations, the dolomite in the Dengying Formation of Ediacaran was selected as an example. Five fabrics (abiogenic and biogenic micrites, dolomitized ooids and early cement, and saddle dolomite) were identified in this Ediacaran-aged formation and they show a large variation in delta 13C values (from 0 to 6 ppm). Based on clumped isotopes (Delta 47) and strontium contents, this study calculated the degree of fluid-rock interaction (W/R ratio) and established a diagenetic framework for each fabric. Both the abiogenic and biogenic micrites were characterized by the rock-buffered system (low W/R ratio = 0.1-0.2), while the fabrics of the ooid, early cement and saddle dolomite were buffered by the hydrothermal fluids in the late diagenesis (high W/R = 1.4-5). Furthermore, this interpretation is further supported by high delta 26Mg values, Sr and Mn contents and elevated delta 34SCAS values in biogenic micrite indicating a closed or partly closed diagenetic system which allow the Rayleigh distillation enriching (super 26) Mg and enhanced (super 34) S of residual SO (sub 4) (super 2-) by bacterial sulphate reduction. In contrast, the fabrics (ooids, early cement and saddle dolomite) in the hydrothermal-buffered system showed an extremely low values indelta 34SCAS, which can be interpreted as a result of oxidation of sulphides (e.g. pyrite). The outcome of this study highlights the utility of systemically petrographic analysis and multiple geochemical proxies to decipher the diagenetic processes of dolomite and caution on interpreting the geochemical records in dolomite successions

Reconstructing paleoceanographic conditions during the middle Ediacaran: Evidence from giant ooids in South China

The Ediacaran was a period of intense environmental change in Earth history. Transient pulses of oxygenation and shifts in ocean chemistry have been interpreted from a range of geochemical proxies, including rare earth elements and yttrium (REY), but it is still a challenge to reconstruct paleoceanographic conditions due to the scarcity of reliable recorder. In this study, we explore the possibility that a newly-discovered deposit of giant ooids in the Nanshanping outcrop in South China contains a relatively intact record of the REE composition of Ediacaran seawater, providing an important window into the redox state of the surface ocean at this time. These ooids are relatively large in size (similar to 1-6 mm), and exhibit densely banded laminae. A regional chemostratigraphic correlation shows that the giant-ooid-bearing succession in South China is in the upper part of the second Ediacaran positive carbon isotopic shift (EP2), and thus its occurrence was not associated with the Shuram Excursion. Additionally, the occurrences of giant-ooid-bearing oolites in different blocks were not synchronous during the middle Ediacaran. In-situ laser ablation analysis of giant-ooid cortices reveals that the shale-normalized REY patterns are similar to those of modern seawater, including depleted light REEs, superchondritic Y/Ho ratios, and positive La anomalies. Yet, no Ce anomalies (1.0 +/- 0.15) in numerous ooid cortices seems to indicate a suboxidized condition of surface ocean in South China at that time. The petrographic and geochemical evidence support the weak diagenetic alteration of the pure giant ooids during burial. They formed in open-ocean settings with energetic hydrodynamic environments and were affected by early dolomitization that might preserve mostly original petrographic fabrics and REY signatures from further diagenetic overprinting. The characteristics of Ce anomalies and REY compositions acquired from the giant ooids indicate that the surface ocean was suboxidized in South China during the late stage of EP2, implying the very low atmospheric O-2 level at that time. Our results suggest that giant ooids may be a reliable analytical target that provide a snapshot into palaeoceanographic conditions during this crucial period of dynamic changes in early Earth history

Impacts of different matrix components on multi-scale pore structure and reservoir capacity: Insights from the Jurassic Da’anzhai member in the Yuanba area, Sichuan Basin

Pore structure is an important parameter for the gas-bearing evaluation of lacustrine shale. However, the characteristics and influencing factors of pore structure and the shale reservoir capacity remain unclear due to the heterogeneity and complexity of shale reservoirs. This study focused on Da’anzhai shale to research the pore structure characteristics using a combination of field emission scanning electron microscopy, low pressure gas adsorption, and mercury intrusion porosimetry. The effects of different matrix components on multi-scale pore structure and the reservoir capacity are discussed. The results show that the total organic carbon (TOC) and vitrinite reflectance (Ro) are 0.7%–1.68% (average of 0.98%) and 1.42%, respectively. The organic macerals are vitrinite, inertinite, and solid bitumen, with the vitrinite predominating. The main minerals are clay minerals, quartz, and calcite. Matrix pores, including mineral-associated pores and organic matter pores, and microfractures jointly constitute the pore system of the Da’anzhai shale. Clay-mineral pores dominate, followed by organic matter pores. Pore sizes mainly range from 0.3 nm to 20 nm, reaching up to 10μm. Mesopore volumes are dominant, followed by micropores and macropores. The pore specific surface areas are 12.76–21.24 m2/g (average of 17.43 m2/g), of which micropores and mesopores account for 48.7% and 51.3%, respectively. TOC, organic macerals, and clay minerals are the controlling factors of the pore structure. In contrast, the impacts of Ro and the framework mineral content on shale pore structure are relatively weak. Da’anzhai shale has sufficient pore volume and specific surface area but the shale is dominated by clay mineral-associated large-size pores and microfractures with few organic micropores, resulting in limited adsorption capacity for the shale. A greater understanding of the pore formation mechanism of shale reservoirs is obtained through this study, which is of great significance for gas-bearing evaluations

Coupling relationship between reservoir diagenesis and gas accumulation in Xujiahe Formation of Yuanba–Tongnanba area, Sichuan Basin, China

The relationship between reservoir tightening time and gas charge period are the key subjects that have not been well solved considering the studies on the tight sand gas accumulation mechanism and enrichment regularity. The diagenetic evolution history, interaction sequence of organic–inorganic in aquiferous rock, gas charge history, the tightening mechanism of tight sandstone reservoir and the relationship between reservoir tightening time and gas accumulation period of the Xujiahe Formation have been analyzed in the Yuanba–Tongnanba area of the Sichuan Basin. It has been confirmed that the main reason for the tight sandstone reservoir formation is the intensive mechanical compaction which has dramatically reduced the sandstone reservoir quality, and it resulted to a semi-closed to a closed diagenetic fluid system formation at the early diagenetic stage. In the semi-closed to a closed diagenetic fluid system, at the later part of the diagenetic stage, the fluid circulation is not smooth, and the migration of the dissolution products are blocked, hence, the dissolution products mainly undergo the in situ precipitation and cementation. Such dissolution products block the dissolution pores and the residual primary pores; and the stronger the dissolution is, the higher the cement content is, which makes the reservoir further tightened. The hydrocarbon generation and expulsion history of source rocks and reservoir fluid inclusion characteristics in the Xujiahe Formation show that the charge of natural gas occurs in the Middle Jurassic–Early Cretaceous (mainly Early Cretaceous). A comprehensive analysis of the reservoir tightening history, gas charge history, and interaction sequence of organic–inorganic aquiferous in rock indicate that the sandstone reservoir experienced a tightening process when gas charging took place in the Xujiahe Formation in the Yuanba–Tongnanba area of the Sichuan Basin

Tracking hydrological processes during the formation of Marinoan cap dolostone; insight from reordered clumped isotopes

Widespread dolomite precipitates, known as cap dolostone, were deposited in the aftermath of the Neoproterozoic Marinoan glaciation, and are thought to reflect dramatic perturbations of seawater chemistry in the aftermath of the glaciation. However, controversy still remains on the nature and the hydrodynamic process of fluids that produced these dolomites. In this study, we present clumped isotope values (Delta (sub 47) ) of Neoproterozoic Marinoan cap carbonates from five sections in Namibia and South China and examine the spatial variation of the post-glacial hydrodynamics across the two carbonate platforms. The Delta (sub 47) -temperatures show the mineral-specific variation in dolostone and limestone, a feature that can be attributed to the solid-state reordering driven by short-term hydrothermal events. Based on the results of theoretical calculations and laboratory experiments, we present a framework that enables the utilization of the reordered clumped isotopes to trace the dolomitization process. Under such a framework, we find the reordering patterns determined by clumped isotopes are indistinguishable from the inner to outer platforms as well as between the dolostone and limestone, which suggests the mixture of meltwater and seawater as the source of dolomitizing fluids. Our findings demonstrate the clumped isotope can be used to trace hydrological processes for deep-time successions, and offer insight into the enigmatic origin of cap carbonates

A clumped isotope diagenetic framework for the Ediacaran dolomites: Insights to fabric‐specific geochemical variabilities

ABSTRACT While marine dolomites formed under near surface conditions have been considered to be potentially reliable archives of past oceanic conditions, this interpretation comes with significant challenges because diagenetic alteration frequently produces diverse fabrics with large geochemical variability. It has been suggested that the Ediacaran dolomites in South China (Hamajing Member, Dengying Formation) recorded the oceanic conditions present at the time they formed, yet these dolomites are composed of five different fabrics (stromatolitic, micritic, oolitic, saddle dolomites and fibrous–radial dolomite cements) and show large variations in multiple geochemical isotope proxies (carbon, oxygen, clumped, magnesium and the sulphur of carbonate‐associated sulphate). This study establishes a paragenetic sequence for these dolomites by combining the clumped and the oxygen isotopic compositions, thereby assessing whether they are geochemically representative of the original seawater. Using this diagenetic framework, the micritic and stromatolitic dolomites show a closed‐system behaviour (low water–rock ratios; 1). Furthermore, in a closed‐system environment, the elevated δ 24 Mg and δ 34 S values in the stromatolitic dolomite reflect the isotopic Rayleigh fractionation that enriches the 26 Mg and 34 S through rock‐buffered recrystallization, coupled with microbial sulphate reduction. These results demonstrate that the complex signals in early marine dolomite should be carefully evaluated when used as a palaeoproxy

Using clumped isotopes to determine the origin of the Middle Permian Qixia Formation dolostone, NW Sichuan Basin, China

The dolostone reservoir in the Middle Permain Qixia Formation (P₂q) is a new region of natural gas exploration in the Sichuan Basin. The formation temperature, the δ¹⁸O of the dolomitization fluid, and the origins of the two types of dolostone in the northwestern Sichuan Basin are discussed, using new carbonate clumped isotope data and conventional petrology and geochemical analyses. The massive dolostone (MD) is composed of coarse anhedral dolomite, whereas the leopard dolostone (LD) is composed of euhedral and subhedral dolomite. A formation temperature of 56.7 ± 3.8 °C for the MD was determined using carbonate clumped isotopes. This is significantly higher than the burial temperature (about 45 °C) with coeval geothermal gradient. The δ¹⁸O_(VSMOW) of the dolomitization fluid is −4.0 ± 0.7‰, which is about 2 permil lower than that of the Middle Permian seawater. The Fe and Mn concentrations, ∑REE abundances, δ¹³C values, and ⁸⁷Sr/⁸⁶Sr ratios of the MD are higher, lower, lighter, and heavier, respectively, than those of the coexisting limestone. These findings indicate that the dolomitization fluid for MD was meteoric water modified-Middle Permian saewater. Thus, we conclude that the MD was formed by meteoric-water-modified seawater replacement in shallow burial condition, but it experienced abnormally high temperatures, resulting from a basaltic eruption at the end of the Middle Permian. A formation temperature of 65–72 °C was determined for the LD using clumped isotopes. This is also significantly higher than the burial temperature with coeval geothermal gradient. The δ¹⁸O_(VSMOW) of the dolomitization fluid is approximately +2.0‰, which is significantly higher than that of the Middle Permian seawater (−2.2‰ ~ −1.0‰). The trace element abundances of the LD are higher than those of the coexisting limestone, and the REE patterns of the LD are similar to that of hydrothermal dolostone in the southwestern part of the Sichuan Basin. Thus, we conclude that the LD was formed by hydrothermal replacement at shallow depths. During the hydrothermal replacement process, the pre-existing MD was also altered by the hydrothermal fluid and dissolution vugs were produced, which improved the reservoir's porosity and permeability

Using clumped isotopes to determine the origin of the Middle Permian Qixia Formation dolostone, NW Sichuan Basin, China

The dolostone reservoir in the Middle Permain Qixia Formation (P₂q) is a new region of natural gas exploration in the Sichuan Basin. The formation temperature, the δ¹⁸O of the dolomitization fluid, and the origins of the two types of dolostone in the northwestern Sichuan Basin are discussed, using new carbonate clumped isotope data and conventional petrology and geochemical analyses. The massive dolostone (MD) is composed of coarse anhedral dolomite, whereas the leopard dolostone (LD) is composed of euhedral and subhedral dolomite. A formation temperature of 56.7 ± 3.8 °C for the MD was determined using carbonate clumped isotopes. This is significantly higher than the burial temperature (about 45 °C) with coeval geothermal gradient. The δ¹⁸O_(VSMOW) of the dolomitization fluid is −4.0 ± 0.7‰, which is about 2 permil lower than that of the Middle Permian seawater. The Fe and Mn concentrations, ∑REE abundances, δ¹³C values, and ⁸⁷Sr/⁸⁶Sr ratios of the MD are higher, lower, lighter, and heavier, respectively, than those of the coexisting limestone. These findings indicate that the dolomitization fluid for MD was meteoric water modified-Middle Permian saewater. Thus, we conclude that the MD was formed by meteoric-water-modified seawater replacement in shallow burial condition, but it experienced abnormally high temperatures, resulting from a basaltic eruption at the end of the Middle Permian. A formation temperature of 65–72 °C was determined for the LD using clumped isotopes. This is also significantly higher than the burial temperature with coeval geothermal gradient. The δ¹⁸O_(VSMOW) of the dolomitization fluid is approximately +2.0‰, which is significantly higher than that of the Middle Permian seawater (−2.2‰ ~ −1.0‰). The trace element abundances of the LD are higher than those of the coexisting limestone, and the REE patterns of the LD are similar to that of hydrothermal dolostone in the southwestern part of the Sichuan Basin. Thus, we conclude that the LD was formed by hydrothermal replacement at shallow depths. During the hydrothermal replacement process, the pre-existing MD was also altered by the hydrothermal fluid and dissolution vugs were produced, which improved the reservoir's porosity and permeability