Shale Gas Exploration Area Research Articles

Direct inversion for shale brittleness index using a delayed rejection adaptive Metropolis-Markov chain Monte Carlo method

The reservoir brittleness index can characterize the relative brittleness of shale oil and gas reservoirs, which provides guidance for hydraulic fracturing in the later stage of oil and gas exploration and development. It is a significant index to evaluate the sweet spot of oil and gas reservoirs. Most research on the brittleness index involves laboratory measurements and petrophysical quantitative analysis, but few studies directly predict the reservoir brittleness index from prestack seismic data. The Markov chain Monte Carlo (MCMC) probabilistic algorithm incorporating the delayed rejection adaptive Metropolis (DRAM) strategy has been developed to invert the reservoir brittleness index directly. The different brittleness indexes are analyzed and compared by using the constructed rock-physics model of the shale gas exploration area to select the most sensitive one. Based on the chosen brittleness index, the approximate equation of the PP-wave reflection coefficient for the brittleness index is deduced, and the correctness and achievability of the modeling operator are indicated by theoretical analysis. To analyze the uncertainty of the brittleness index and enhance computational efficiency, an improved MCMC probabilistic inversion algorithm is introduced, which aims to optimize the sampling process of the general MCMC algorithm by delayed rejection and adaptive Metropolis strategies. Our study finds that the DRAM strategy improves the efficiency of the algorithm by nearly 30%. Synthetic examples and field seismic data verify the applicability and effectiveness of our inversion method.

Provenance and Tectonic Setting Discrimination and Their Effects on the Pore-Scale Fluid Flow of Black Shale: Take the Micangshan Tectonic Belt, Central China, as an Example

The Lower Paleozoic reservoir in the Micangshan tectonic belt is a new shale gas exploration area with excellent potential. However, the black shale’s sedimentary environment and tectonic setting and their effects on pore-scale fluid flow have not been thoroughly elucidated and urgently require further research. A large amount of trace and rare earth element data of black shale in this area was tested to analyze the sediment’s provenance, tectonic setting, and coupling process, and nuclear magnetic resonance was applied for fluid flow ability determination. The characteristics of trace elements and rare earth elements in shale samples were drilled from the Upper Sinian Doushantuo Formation, the Lower Cambrian Niutitang Formation, and the Lower Silurian Longmaxi Formation in the Micangshan tectonic belt. The results reveal that the provenance of the shales was dominated by island arc volcanic rocks and some ancient fine-grained sediments. The provenances of shale in the Micangshan, Huijunba, and Zhenba areas may have a strong genetic relationship with the igneous rocks from the nearest Hannan massif. The provenance of the Niutitang shale in the Ningqiang area may comprise sediments derived from the Longmenshan continental rift system. The provenance of the shale tends to transition from the island arc to the continental margin from the depositional stage of the Doushantuo Formation to the Longmaxi Formation. The Micangshan tectonic belt inherited the tectonic evolution of the South China block, North China block, and Qinling, and those blocks provided the igneous provenance of the shale by island arcs, which were formed due to weathering caused by the process of subduction and collision orogeny. The movable fluid distribution of the Longmaxi Formation and Niutitang Formation is relatively uniform, and excessive pursuit of large amount of movable fluid will be counterproductive to the shale reservoir. The proportion of the cracks was generally less than 3% except for one sample from the Doushantuo Formation with 12.06%. Based on the centrifugation force in this research, the low fluid flow limits were less than 1.47, and there are strong positive relationships between movable fluid saturation and saturated porosity. In this study, the coupling relationship between the material source and pore scale flow is discussed for the first time, and the results can provide theoretical references for local provenance analysis and tectonic movement.

Analysis on Black Shale Sedimentary Paleoenvironment and Tectonic Setting of the First Member of Luzhai Formation of Lower Carboniferous, Rongshui Area of Guangxi, China

In order to promote the discovery of new shale gas exploration areas in Guangxi, based on the latest drilling data, through detailed observation and description of logging data and cores, this paper selects high-quality shale samples of the target layer for X-ray diffraction analysis and testing of clay minerals and whole rock, major and trace elements (including rare-earth elements), TOC, and other related analysis and testing. The study shows that the total amount of rare-earth elements in the first member of Lower Carboniferous Luzhai Formation in Rongshui area, Guangxi, is low; δ EU and δ CE show negative abnormality. The average value of w V / V + Ni , Th / U ratio, and Mo-TOC covariant model diagram of the first member of Luzhai Formation show that it was the condition of anoxic reduction at that time. At the same time, Ba-Sr diagram, Sr/Cu ratio, and total rare-earth elements reflect that the area was in an arid, hot, and marine salt water paleoenvironment during the sedimentary period. Most of the sample parent rocks of the first member of Luzhai Formation are in shale area, and some samples are close to felsic igneous rock area, which is of mixed origin. The tectonic background of the source area is mainly continental island arc and active continental margin, indicating that the sedimentary environment was unstable and highly active under the background of extensional rifting at that time.

Magnetic Anomaly Characteristics and Magnetic Basement Structure in Earthquake-Affected Changning Area of Southern Sichuan Basin, China: A New Perspective from Land-Based Stations

The Changning area is located in the southern Sichuan basin and the western Yangtze Plate and is the most abundant shale gas exploration area in China. In recent years, Changning has experienced frequent earthquakes with moderate magnitudes, attracting extensive interest. To investigate the magnetic characteristics in Changning, 952 land-based stations were employed to establish a magnetic anomaly model with a resolution of 2 km, and the subsurface magnetic basement structure was obtained by an iterative algorithm in the Fourier domain. The magnetic anomaly model shows significant distinctions between the northern salt mine area and the southern shale gas area. The magnetic basement includes the crystalline basement and the Sinian sedimentary rock metamorphic basement, which has strong magnetism. The large intracratonic rift that developed in the Sinian–Early Cambrian plays an important role in the evolution of Changning, which also impacts magnetic anomalies and the magnetic basement structure. Finally, by comparing the seismic wave velocity ratio structure, the deeper magnetic basement that corresponds to the higher seismic wave velocity ratio can be explained. This article implies that magnetic anomalies and magnetic basement depth have a certain correlation with earthquakes in Changning, and it provides a geodynamic reference for Changning and the southern Sichuan basin.

Silica crystallinity: Characteristics and controlling factors in marine shale of the upper Yangtze area, China

The quartz crystallinity index (QCI) was applied to quantitatively characterise the crystallinity features of silica within the marine shale of the upper Yangtze area. A total of 67 samples, including 63 shale samples, 3 quartz vein samples, and 1 artificial quartz sample as reference, were applied in QCI calculation by X-ray diffraction (XRD). Furthermore, based on the identification and classification of graptolite biozones, a series of experimental analyses, including the total organic carbon (TOC) content, bitumen reflectance, and elemental composition, were conducted to determine the silica origin and controlling factors of crystallinity. The results indicated that biogenic silica in marine shale intervals had an obviously lower crystallinity than silica of other origins. The quartz vein in shale intervals presented the highest silica crystallinity. Silica that experienced a low diagenesis intensity exhibited the lowest crystallinity. Furthermore, biogenic silica in a state of overpressure had a much lower crystallinity than that under normal pressure. The crystallinity of silica in marine shale of the upper Yangtze area is affected by origin, diagenesis intensity, and overpressure. From the perspective of QCI, the Chengkou area is probably the next favourable area for shale gas exploration and development.

Characteristics of Karst Formations and Their Significance for Shale Gas Exploration

The classification method of karst formations is widely used in engineering and environmental geology but is seldom used in petroleum geology. In this study, the classification method of karst formations is applied to the sealing study of shale gas roof and floor carbonate rocks, and the influence on shale gas accumulation and drilling is discussed. The Paleozoic black shale in southern China is primarily formed by marine and transitional faces, and the intergrowth between shale and carbonate rocks is a basic geological feature of the Paleozoic strata in southern China. Carbonate karst is an unavoidable problem in shale gas exploration in southern China. Around the black shale target layer, in the Upper Paleozoic trait region, the study starts from the development strength of karst strata, through geological profile survey, spring flow statistics, test, and other methods and means; the shale and the carbonate rock contacted with it are taken as a whole to explore the impact of karst strata on shale gas drilling. The Upper Paleozoic karst strata in the study area were divided into two kinds, four types, and six subtypes. It was determined that the limestone continuous karst strata of the Sidazhai Formation and the second member of the Nandan Formation are the sensitive layers of shale gas drilling, whereas the first number of Nandan and Wuzhishan formations are shale reservoir-forming packers. In addition, a method for evaluating the karst sensitivity of shale gas exploration is summarized. The karst avoidance, karst-sensitive, and karst-insensitive areas for shale gas exploration were divided. Combined with the surface and underground conditions and the basic geological conditions of shale gas not being significantly different, shale gas drilling should avoid the fold core and fault zones.

Geological and Geochemical Characteristics of the Terrestrial Lower Jurassic Shale in Tarim Basin and its Favorable Exploration Areas

Large amounts of conventional gas resources that originated from terrestrial Lower Jurassic shale were discovered in the Tarim Basin, indicating promising exploration prospects for shale gas resources, whereas limited understandings were obtained on the geological and geochemical characteristics of the Lower Jurassic shale. In this study, based on cores of exploration wells and field outcrops, total organic carbon (TOC) pyrolysis, microscopic composition, organic elements, vitrinite reflectance, x-ray diffraction, and methane isothermal adsorption experiments were carried out on Lower Jurassic shale in the Tarim Basin. The results show that the TOC content of shale is relatively high, but the TOC content of shale in different regions is quite different. Among them, the Kuqa Depression is the highest (more than 3%), the Southwest Tarim Basin is second (more than 1%), and the East Tarim Basin is the lowest. The main types of kerogen in shale are mainly type III and type II2. The thermal evolution maturity of shale in different regions is quite different; the highest Ro of shale in the Southwest Tarim Basin is more than 2.0% and that of shale in the Kuqa depression is higher than 1.5%, both of which have entered the stage of high-over-mature evolution, and the shale in the East Tarim Basin is only in the low maturity–mature stage. The overall content of brittle minerals such as quartz in shale is greater than 40%, which has good fracturing properties. The adsorbed gas content of shale is high, but there is a great difference (0.5–4 m3/t), which is related to the large difference in the abundance of organic matter, thermal maturity, and clay mineral content of shale. Therefore, the Lower Jurassic shale gas in the Tarim Basin has good prospects for exploration. Based on the superposition method of the main controlling geological factors, it is predicted that the favorable areas for shale gas exploration are mainly located in the Keshen and Yinan-Yeyun areas of the Kuqa Depression and the Caohu Sag-Mandong area of the East Tarim Basin. This research not only provides basic data for the evaluation of shale gas resources in our country, but also provides certain guidance for the exploration of Lower Jurassic shale gas in the Tarim Basin.

Formation mechanism of the sealing capacity of the roof and floor strata of marine organic-rich shale and shale itself, and its influence on the characteristics of shale gas and organic matter pore development

The sealing capacities of shale, roof, and floor strata are essential for the preservation of shale gas. Sealing capacity development and its influence on shale gas composition and organic matter pore development are important but have not been addressed by existing studies. In this paper, the Upper Ordovician Wufeng-the first member of the Lower Silurian Longmaxi Fm. in the Yangtze region is selected as the study object. We dissected the J1 well in the Jiaoshiba block inside the Sichuan Basin and the Y6 well in the Youyang block outside the Sichuan Basin to analyze the gas components and carbon isotopes in recovered shale gas. In this study, two microscope techniques (i.e., FIB-SEM and FIB-HIM), porosity measurement, TOC content analysis, permeability tests, isothermal adsorption measurements, are utilized. The analytical results show that both the roof and floor strata have vertical sealing effects on the organic-rich shale due to their different physical properties compared to the shale. The shale overburden pressure increases with burial depth, and the adsorbed gas increases with increasing TOC content and thickness: all of these can lead to a decrease in permeability and an increase in self-sealing capacity. This paper determined the formation mechanism of the sealing capacity for shale, roof and floor strata. Furthermore, the control patterns of “sealing capacity strength” on the shale gas source and organic matter pore development are clarified. The main findings are as follows: when the shale, roof, and floor strata have a strong sealing capacity, only a small amount of liquid hydrocarbons can be discharged from the shale, but the natural gas formed due to liquid hydrocarbon cracking is abundant. Under such circumstances, shale gas has a mixed origin of liquid hydrocarbon cracked gas and kerogen cracked gas; the carbon isotopes are inverted; abundant pyrobitumen pores with high porosity and good connectivity are developed. When the sealing capacities are weak, the amount of liquid hydrocarbons discharged from the shale are high, and the amount of natural gas is low. The shale gas is mainly derived from kerogen cracking with weak or no carbon isotope inversion, and kerogen pores are small with poor connectivity. These findings will advance the study of shale gas formation mechanisms and provide a scientific basis for selecting shale gas exploration area.

Geological Characteristics of Deep Shale Gas in the Silurian Longmaxi Formation in the Southern Sichuan Basin, China

To investigate the geological characteristics and exploration potential of shale gas in the southern Sichuan Basin, we analyze the coupling relationship between the hydrocarbon generation and storage conditions of the Longmaxi Formation and discuss the preservation conditions from the lateral and vertical migration mechanisms of shale gas. According to the results, the organic-rich shale at the bottom was formed in a strongly euxinic and anoxic reducing environment, the oxygen content increased in the upper water body in which the Longmaxi Formation was deposited, and the water body became oxidized. The organic matter type in the shale is dominantly type I kerogen and a small amount of type II1 kerogen. The organic matter content is more than 3.0% and is in the high-to postmature stage. The enrichment of siliceous organisms increases the organic matter and enhances the brittleness of shale, resulting in “superior hydrocarbons and a favorable reservoir”. Pyrolysis of organic matter promotes the formation of organic matter pores and dissolution pores, resulting in a coupled “source-reservoir” accumulation control system. The high vertical formation pressure guarantees the sealing of shale and restrains the lateral escape of shale gas. The high-angle intersection of the highly filled fractures and the current crustal stress can effectively enhance the fracture sealing and inhibit the vertical escape of shale gas, forming a three-dimensional effective closure. Hence, the area featuring a short tectonic uplift time, small amplitudes, large-scale underdeveloped fractures, and a high formation pressure coefficient is a favorable area for shale gas exploration, according to the analysis of three-dimensional preservation conditions.

Evaluation of the Accumulation Conditions and Favorable Areas of Shale Gas in the Upper Palaeozoic Marine-Continental Transitional Facies in the Daning-Jixian Area, Ordos Basin

This study focuses on the organic-rich mud shale in the Upper Palaeozoic transitional facies in the southeastern margin of the Ordos Basin. It systematically analyzes the shale gas accumulation conditions of the organic-rich mud shale in the Lower Permian Shanxi-Taiyuan Formation, including the thickness, distribution, organic matter type and content, thermal maturity, reservoir space, gas-bearing property, and rock brittleness. The results show that the thick dark mud shale contains a high organic matter content, is a suitable kerogen type for gas generation, and exhibits moderate thermal evolution, providing excellent conditions for hydrocarbon accumulation. Residual primary pores formed by shale compaction, secondary pores formed by organic matter hydrocarbon generation, clay mineral transformation and dissolution, and fractures provide suitable reservoir spaces for shale gas. The shale in the study area has a higher gas content than the shale strata in the marine basins of the United States. In addition, the content of brittle minerals such as quartz is higher, and Poisson’s ratio is lower, facilitating the subsequent transformation. The accumulation conditions indicate the high potential of the study area for shale gas exploration and development. The geological analogy method is used to compare the study area with five major shale gas basins in the United States. The results indicate that the shale gas resources of the Shanxi-Taiyuan Formation in the study area are in the range of 2800– 3200 × 10 8 m3. The primary controlling factors affecting shale gas reservoirs in this area are the abundance of organic matter, thermal maturity, shale thickness, and quartz content. Favorable areas are predicted based on these factors.

Shale gas enrichment model and exploration implications in the mountainous complex structural area along the southwestern margin of the Sichuan Basin: A new shale gas area

Marine shale gas in the southern Sichuan Basin is the most successful area of shale gas exploration and development in China. In order to open up new shale gas fields and search for new shale gas reserves and production replacement blocks, it is necessary to continuously establish and complete the standards on shale gas reservoir evaluation and area selection under different structural settings. The early exploration practice shows that shale in the mountainous complex structural area along the southwestern margin of the Sichuan Basin varies greatly in the gas bearing property, so systematical analysis and study on the shale gas enrichment mechanisms in this area is of great significance to searching for new shale gas exploration areas. Based on drilling data of 5 wells in the mountainous complex structural area along the southwestern margin of the Sichuan Basin, the main factors controlling the shale gas bearing property and the shale gas enrichment model were discussed based on the systematical experiments and analysis of the outcrops, cores and cuttings of Longmaxi Formation shale from the aspects of organic geochemistry, physical property, gas content, hydrocarbon generation history and pore evolution history, so as to lay a solid geological basis for the birth of a new shale gas area. And the following research results were obtained. First, the shale of the first submember of first Member of Longmaxi Formation (S1l11) in the mountainous complex structural area along the southwestern margin of the Sichuan Basin has an average TOC of 3.02%–4.97% and Ro of 2.38%–3.37%, and the average total gas content in local enrichment zones is up to 4.62 m3/t, so it is classified as quality shale. Second, the detailed studies on hydrocarbon generation history indicate that the shale has the characteristics of “low thermal evolution rate and low maturity”. It is characterized by late hydrocarbon generation, low thermal evolution rate, low current maturity and short late diffusion time, which are favorable for shale gas enrichment. Third, the pore evolution history reveals that shale pore evolution can be divided into six stages, i.e., sharp pore reduction, pore reduction, organic pore formation, pore preservation, organic pore dissipation and karstification, and organic pore and TOC are the most direct control factors of shale gas content. Fourth, favorable shelf facies belt is conducive to the formation of large-scale reservoir space and effective pores, and diversities of preservation conditions under different structural styles and at different structural positions control different pore evolution stages. The shale gas in the mountainous complex structural area is characterized by horizontal zoning and differential enrichment. “Low thermal evolution rate and low maturity” and “slow structural uplifting” are beneficial to the long-term enrichment and preservation of shale gas. In conclusion, the mountainous complex structural area along the southwestern margin of the Sichuan Basin has a shale gas enrichment model of “sedimentation controlling source rocks, diagenesis controlling reservoirs and structure controlling preservation”. This research result provides idea and reference for searching for new shale gas areas and fields.

Geological conditions and exploration potential for shale gas in Upper Permian Wujiaping Formation in the region of western Hubei-eastern Chongqing

The Upper Permian Wujiaping Formation shale in the region of western Hubei and eastern Chongqing has been found qualified for shale gas accumulation in early exploration activities, but the potential for shale gas exploration is still unclear. Based on the core analysis and test data of well Jy3, combined with regional seismic, testing and logging data of this well and wells nearby, the shale distribution, quality, gas-bearing properties, preservation conditions and resource scale of the second member of Wujiaping Formation were discussed. The results showed that the organic-rich shale of deep-water shelf facies and slope to shallow shelf facies were developed and they appeared to be thin in the southwest and thick in the northeast. The shale was mainly composed of quartz (averages 43.10%), clay minerals (average 21.95%) and calcite (averages 17.20%). The organic matter is rich (average TOC value of 8.77%), mainly of type Ⅱ1 kerogen, high thermal maturity (average converted %Ro value of 2.06%), excellent porosity (average value of 5.59%) and good gas bearing property (total gas content averages 3.73 m3/t). The Wujiaping Formation in the Hongxing area is the most promising replacement strata and area for shale gas exploration and development, which shares the largest shale thickness, the best preservation condition, and the most abundant resource.

Influence of interbedded rock association and fracture characteristics on gas accumulation in the lower Silurian Shiniulan formation, Northern Guizhou Province

Abstract The northern Guizhou area, located near the southwestern margin of the Yangtze Block, is a promising area for shale gas exploration and development. The Lower Silurian Shiniulan Formation as a new discovery stratum of natural gas marks an exciting breakthrough in natural gas exploration in northern Guizhou area. Based on several field investigations and samples analyses, the lithology and fracture characteristics were systematically analyzed in the lower Shiniulan Formation, and the reservoir specificity and its influence on natural gas accumulation were determined. The characteristics of the relatively fractures and lithology assemblages were identified as key factors controlling the natural gas accumulation. The lower Shiniulan Formation is deposited as calcareous shale and marlstone with frequent centimeter-scale interlayers. This is reflective of a shallow sea shelf strata with decreasing sedimentary rhythm and gradual weakening of sedimentary changes and developed calcareous shale and marlstone with frequent centimeter scale interlayer changes. The gas reservoir is dominated by calcareous mudstone, controlled by the interbedded rock association (calcareous mudstone and limestone), characterized by the raw-storage and the accumulation-reservoir interbedded system. The reservoir is located in the central part of the syncline and is characterized by strong sealing of the stratum, large proportion of free gas, and high abnormal pressure. The Lower Shiniulan Formation is formed between the shale layer with horizontal fractures and dense limestone with underdeveloped fractures. Among them, the shale section generally develops diagenetic shrinkage fractures, which provide good storage space for natural gas and act as the main body of natural gas. The pore sizes in limestone (2.8 nm) are significantly smaller than those in mudstone (7.5 nm), which results in a good capping and preservation of shale gas. This paper reports on results that are of significance for supplementing the theory of unconventional natural gas accumulation and guiding shale gas exploration in similar areas.

A novel approach of evaluating favorable areas for shale gas exploration based on regional geological survey and remote sensing data

The periphery of the Huangling Massif is one of the principal prospecting regions of the Lower Cambrian shale gas resources in South China. The geological setting and ground surface conditions are complicated within this area, and the petroleum geological data is limited due to the low degree of exploration. A novel approach based on the regional geological survey and remote sensing data was proposed. The favorable areas for gas exploration targeting the Lower Cambrian Niutitang Shale within the periphery of the Huangling Massif were evaluated by using this approach. Seven key indicators, including the depositional facies and current burial state of the shale strata, density of the exposed faults, dip angle of exposed strata, elevation and slope of the current ground surface, and land utilization, were selected. An evaluation criterion of these indicators was established to facilitate the classification of the study area into four ratings: favorable, low-risk, high-risk, and unfavorable. The overall evaluation of the seven indicators suggested a favorable area of 386.02 km2, a low-risk area of 1784.81 km2, and a high-risk area of 1689.97 km2, accounting for 0.91%, 4.21%, and 3.99% of the whole area, respectively. These results achieved a high level of compliance with the actual exploration discoveries and eliminated over 90% of the total area unsuitable for exploration and production. It is demonstrated that this novel approach can be effectively adopted in the comprehensive evaluation of the favorable area for shale gas exploration with a low degree of prospecting.

Organic matter accumulation of the early Cambrian black shales on the flank of Micangshan-Hannan Uplift, northern upper Yangtze Block, South China

The Micangshan-Hannan Uplift area is proved a new shale gas exploration area with massive potential of Niutitang Formation in the past three years. Nevertheless, the organic matter enrichment mechanisms of this set of organic-rich shales lack discussion. Here, eighty-two samples of a drilling well from the Micangshan-Hannan Uplift, Upper Yangtze Block, South China were investigated for the organic matter (OM) accumulation mechanisms by geochemical analysis. Specifically, paleoredox conditions are classified by V/Sc, U/Th, and Uau; primary productivity is evaluated by TOC, Cu/Al, Ni/Al and, P/Al; sediment flux is indicated by Ti/Al; and paleoclimate is inferred by CIA and Ga/Rb ratios. Vertically, the Kuanchuanpu dolomite, underlying the Niutitang black shales, with TOC contents from 0.34% to 1.18% (avg. 0.71%), was deposited in oxic and low productivity conditions under lower sea level and more humid climate. Upwards, the silica-dominated and clay-rich siliceous mudstone (Assemblage 1) of Niutitang Formation, characterized by high TOC contents from 1.31% to 4.66% (avg. 2.78%) was classified into three units. The OM enrichment of Unit 1 was controlled by both anoxic condition and high primary productivity resulting from rapid sea level rise and increasing aeolian flux under cold and arid climate. The OM enrichment of Unit 2 was manipulated by the dysoxic bottom water condition, although the primary productivity remained relatively high. Then, the OM enrichment of Unit 3 was dominated by increasing primary productivity of surface water due to higher aeolian input and the dysoxic with intermittent anoxic condition advantageous for OM preservation. At last, the mixed siliceous mudstone (Assemblage 2) of Niutitang Formation displayed much lower TOC contents (avg. 1.34%). Both of oxic condition as well as lower primary productivity due to the declining aeolian flux led to the less efficient OM accumulation. In sum, the favorable organic matter accumulation in Niutitang Formation was controlled by anoxic bottom water condition and high primary productivity, which was related to the Early Cambrian global transgression and increasing aeolian flux under cold and arid climate.

Analysis of Developmental Characteristics and Dominant Factors of Pore-Fracture Systems in Lower Cambrian Marine Shale Reservoirs: A Case Study of the Niutitang Formation, Fenggang Block, Southern China.

Due to breakthroughs in the Lower Silurian Longmaxi Formation in the Sichuan Basin and multiple strata around the basin, the northern part of Guizhou adjacent to the Sichuan Basin has become a key area for shale gas exploration. Compared with the Longmaxi Formation, the Niutitang Formation displays greater TOC (total organic carbon) content, depositional thickness and distribution area, but the details remain undetermined. In the study area, the Lower Cambrian Niutitang Formation typically has high TOC content, maturity and brittle mineral content. The study area has experienced multiple periods of tectonic movement, which have great influence on the fracture and pore characteristics. The fractures are mainly structural fractures and have obvious zoning. The primary types of pores are intraparticle pores, organic matter pores, and interparticle pores. Further, macropores and mesopores less than 50 nm contribute most of the pore volume, while pores less than 2 nm contribute most of the specific surface area. Many factors affect the pore-fracture system, such as tectonism, TOC content, mineral composition, and sedimentary environment. Tectonic movements produce fractures based on the changing stress field, but the degree of fracture development does not agree well with the degree of pore development. The TOC content has good positive correlations with the development of fractures and micropores, especially for nanoporosity, while clay minerals show a negative correlation with the development of fractures but a strong positive correlation with the growth of micropores. Quartz displays a positive correlation with the development of fractures but no good correlation with pore development. Finally, the lithofacies, lithologies and mineral compositions under the control of sedimentary environments are internal factors that can impact the development of pore-fracture systems.

Mechanical properties of organic-rich shales based on nanoindentation: A case study of deep shale gas reservoirs in Zigong area, Sichuan Basin

The deep shale gas in the Zigong area of the Sichuan Basin has a wide distribution and has good prospects for exploration, which has become an important replacement area for shale gas exploration and development. In this area, the burial depth of Longmaxi Formation Shale is generally >3500 m. One of the most challenging tasks in the development of this type of reservoir is the accurate prediction of the mechanical properties of organic-rich shale. The application of the mechanical properties of shale is important to the success in multiple aspects of drilling, hydraulic fracturing, and production. Methods such as unconfined/triaxial compression tests with the use of core plugs and logging data interpretation are commonly used to obtain mechanical properties. However, these methods have their own limitations. Thus, to address the difficulty in the measurement of the mechanical properties of shale, the advanced nanoindentation test is introduced and used. Nanoindentation is a technique based on a hard tip, which is pressed into the surface of a sample. The advantage of this test is the use of a sample with a small volume, such as drill cuttings or fragments, which are easily obtained during drilling. The mechanical properties at the nanoscale level, including the hardness, Young’s modulus, and fracture toughness, are measured in the nanoindentation. Moreover, this technique can be used to evaluate the creep behavior of shale. In this study, grid nanoindentation is used to investigate the mechanical properties of drill cutting samples from the Longmaxi Formation Shale in Zigong area, Sichuan Basin. Subsequently, these mechanical parameters are used to evaluate the heterogeneous properties of shale at the nanoscale level. The results can help in analyzing the correlation between rock mechanical properties and shale constituents. This suggests that the nanoindentation test is a useful approach of estimating the elastic and mechanical properties of shale in situations where conventional rock mechanics test data cannot be measured.

Patterns of plant species richness and growth form diversity in critical habitats of the Nama-Karoo Biome, South Africa

The distribution patterns and determinants of biodiversity in the Nama-Karoo Biome are poorly known. This poses a problem for conservation given the impact that future developments (e.g. uranium mining, fracking) and global climate change are likely to have on the region's biodiversity. This study investigated the potential role that several environmental and climatic variables play in determining species richness, relative cover, and growth form diversity in two habitat types: plains and rocky dolerite hillslopes. The study area extended over a 600 km longitudinal environmental gradient in the Upper Karoo in which mean annual precipitation varied from 100 to 600 mm from west to east. A pairwise floristic survey approach was implemented, using modified Whittaker plots in each habitat type at 30 sites within the shale gas exploration area in the Upper Karoo bioregion. The results showed that total species richness was significantly higher (p < 0.01) in slope habitats than in plains habitats across the environmental gradient. The greatest diversity in both habitat types in terms of species richness and relative cover was in the low woody shrubs growth form. Similar plant associations on slopes aggregated spatially and conformed closely to their respective vegetation types, while those in plains habitats were interspersed with no clear clustering. A combination of soil, environmental and climatic parameters emerged as significant explanatory variables for modelling total species richness and relative cover of the five dominant growth forms. Habitat type was a significant explanatory variable for total species richness and relative cover. In the current vegetation map of South Africa, the vegetation types for the Upper Karoo bioregion are coarse, giving the impression of homogeneity in what is a highly heterogeneous landscape. Further mapping of fine-scale biodiversity patterns is needed in order to better inform conservation and management practices in the Nama-Karoo Biome.

Sedimentological and geochemical characterization of the Upper Permian transitional facies of the Longtan Formation, northern Guizhou Province, southwest China: Insights into paleo-environmental conditions and organic matter accumulation mechanisms

Characterized by high TOC content and large cumulative thickness, the Upper Permian marine-continental transitional facies of shales are widely distributed in southwest China, and are currently becoming a prospective area for shale gas exploration and production. Compared to marine shales, the organic matter (OM) accumulation model and paleo-environmental conditions remain unclear in transitional settings. In this study, core descriptions, thin section and scanning electron microscopy (SEM) observations, X-ray diffraction (XRD) mineralogical analysis, and a major, trace, and rare earth element analysis were integrated to document the varying lithofacies, paleo-environmental conditions, and OM accumulation mechanisms for the Upper Permian transitional facies of the Longtan Formation in northern Guizhou Province, southwest China.Six major lithofacies, including carbonaceous mudstones, siliceous-clay mixed mudstones, argillaceous mudstones, silty shales, muddy siltstones, and calcareous-clay mixed shales, were identified. The XRD analysis and SEM images suggest that clay (average = 51.2 wt%) and siderite (average = 17.3 wt%) are the dominant minerals in the Longtan Formation, which is different from the mineralogical composition of typical marine shales, which are generally dominated by quartz (e.g. Horn River Shale in the Western Canada Sedimentary Basin, and the Barnett Shale in the Fort Worth Basin). Furthermore, the sedimentological and geochemical analyses suggest that the paleo-environmental conditions and OM accumulation mechanisms of the transitional facies of the Longtan Formation are different from those of marine shales, and can be summarized as follows: (1) terrestrial plant fragments mainly contribute to the TOC contents of the Longtan Formation; (2) paleoproductivity is generally the major influencing factor of OM accumulation in marine shales, but is relatively low during the deposition of the Longtan Formation; (3) terrestrial OM input, redox conditions and sedimentation rate are the major factors controlling OM accumulation; and (4) most of the silica content has detrital origin instead of biogenic origin in the Longtan Formation.

Hydrocarbon migration and accumulation in the Lower Cambrian to Neoproterozoic reservoirs in the Micangshan tectonic zone, China: New evidence of fluid inclusions

The Lower Paleozoic reservoir in the Micangshan tectonic zone is a new shale gas exploration area with excellent potential. However, the hydrocarbon migration and accumulation histories of this reservoir have not been thoroughly elucidated and urgently require further research. Fluid inclusions offer a unique and useful method to test for hydrocarbon migration, accumulation, composition and timing. This study integrated the hydrocarbon charge history using fluid inclusions in the Lower Cambrian to Neoproterozoic reservoir from the Micangshan tectonic zone. This work involves the delineation and analysis of fluid inclusions using the petrography, spectroscopy and microthermometry of fluid inclusions. Based on the fluid inclusion analyses combined with reservoir thermal history, timing estimates and charge models of the hydrocarbons were obtained. The formation of natural gas was multifactorial in the Lower Cambrian to Neoproterozoic petroleum system, and the gas in the Niutitang (ϵ1n) and Dengying (Z2d) reservoirs was mainly from oil cracking and dry gas, and gas in the Xiannvdong (ϵ1x) and Canglangpu (ϵ1c) reservoirs was abiogenic and formed in hydrothermal fluids by regional tectonothermal events. Three well-defined stages of hydrocarbon charge were identified in the petroleum system in the Micangshan tectonic zone. The stage of oil charge first began before the Early Permian (∼277 Ma); the stage of gas charge from oil cracking occurred in the Late Triassic to Early Jurassic (212 Ma-198 Ma); and the stage of gas charge from dry gas occurred in the Middle Jurassic (173 Ma-166 Ma). The gas of mixed origin from the Niutitang reservoir was stored in the nanoscale pores as a self-reservoir, rather than as an effective gas source for the overlying reservoirs.