34 research outputs found
Study on the Sedimentary Environments and Its Implications of Shale Reservoirs for Permian Longtan Formation in the Southeast Sichuan Basin
No full textMarine–continental transitional shale is one of the most promising targets for shale gas exploration in the Lower Yangtze region. To investigate the sedimentary environments and the regularity of the enrichment of the Longtan shale, multiple techniques including core and thin-section observations, geochemical and elemental analyses, X-ray diffraction, scanning electron microscopy (SEM), and low-pressure nitrogen adsorption (LPNA) were used to analyze the sedimentology, mineralogy, and pore structure of the Longtan shale. The core descriptions and thin-section observations showed that the Longtan shale was deposited in marine–delta transitional environments including delta-front, shore swamp, mixed tidal flat and shallow shelf environments. The Sr/Cu, V/Cr, CIA, EF (Mo), EF (U), and other major and trace element results indicated warm and moist climates and water-reducing conditions in the Longtan period. Both the climate and water conditions were favorable for organic matter production and preservation. The geochemical results showed that the Longtan shale was in the overmature stage (Ro values ranging from 2.4% to 3.57%) and that the average total organic carbon (TOC) content was 5.76%. The pore system of the Longtan shale consisted of inorganic pores with a small number of organic pores and microfractures. The porosity and specific surface area were mainly affected by the TOC and clay mineral contents. An effective combination of brittle mineral particles, organic matter, and clay minerals provided the necessary conditions for pore preservation. The organic pores, intergranular pores in clay minerals, and brittle mineral pores formed the main network system for the Longtan shale. In summary, the lithological combinations, organic geochemistry, and pore structure system were all affected by the sedimentary environments
Factors Controlling the Pore Development of Low-Mature Marine–Continental Transitional Shale: A Case Study of the Upper Permian Longtan Shale, Western Guizhou, South China
No full textThe Upper Permian Longtan Shale is a significant reservoir in western Guizhou. To clarify the main factors controlling the low-mature marine–continental transitional shale pore development in western Guizhou, pore types were classified with scanning electron microscopy (SEM), and the pore developmental stages and morphological structures were quantitatively characterized by nitrogen adsorption isotherm analyses. Additionally, the qualitative or semi-quantitative relationships between the pore developmental stages and the main controlling factors were established via geochemical analysis. The results showed that the Longtan Shale pores include intergranular pores, intragranular pores, organic pores, and microfractures. The intergranular pore structures were categorized into ink-bottle, slit, layered, and irregular types. The intragranular pores were found to be of the elliptical, nearly circular, ink-bottle, and irregular varieties. The organic pores were categorized into elliptical, bubble-like, and irregular polygonal variants. The microfractures were only of the elongated type. The clay-mineral-related intergranular pores were the predominant pore type. The organic pores were found to be poorly developed. The mesopores were predominant, followed by macropores. The shale pore diameters ranged between 1 nm and 100 nm, and they are characterized by multiple peaks. The specific surface area (SSA) was primarily provided by nanopores in the range of 5 nm to 10 nm, such that the smaller pores provided a greater contribution to the SSA, and they are more conducive to shale gas adsorption and accumulation. Clay mineral content was the dominant internal factor controlling pore development and the SSA, with the illite–smectite mixed layer being the most obvious controlling factor. While too low or too high clay mineral content is adverse to macropore development, brittle mineral content, carbonate mineral content, and total organic carbon (TOC) content are adverse to pore development and the SSA. Thermal maturity has no remarkable control effect on pore volume and the SSA of non-organic pores
Design and Evaluation of a Surfactant–Mixed Metal Hydroxide-Based Drilling Fluid for Maintaining Wellbore Stability in Coal Measure Strata
No full textCo-exploitation of coal measure gases (coalbed gas, shale gas, and tight sandstone gas) puts a higher requirement on drilling fluids. Conventional drilling fluids have disadvantages, such as causing problems of borehole collapse, formation damage, and water blockage. This paper proposes a set of high inhibitive and low-damage drilling fluids that function by electrical inhibition and neutral wetting. Zeta potential results showed that the negative electrical property of Longtan coal in Bijie, Guizhou, can be reversed by organic mixed metal hydroxide (MMH) and the cationic surfactant alkyl trimethylammonium bromide (CS-5) from −3.63 mV to 19.75 mV and 47.25 mV, respectively. Based on the contact angle and Fourier Transform Infrared Spectroscopy (FT-IR) results, it can be concluded that chemical adsorption dominates between the Longmaxi shale and surfactants, and physical adsorption between the Longtan coal and surfactants. A compound surfactant formula (0.001 wt% CS-4 + 0.001 wt% CS-1 + 0.001 wt% CS-3), which could balance the wettability of the Longmaxi shale and the Longtan coal, making them both appear weakly hydrophilic simultaneously, was optimized. After being treated by the compound surfactants, the contact angles of the Longmaxi shale and the Longtan coal were 89° and 86°, respectively. Pressure transmission tests showed that the optimized combination of compound surfactants and inorganic MMH (MMH-1) could effectively reduce permeability of the Longmaxi shale and the Longtan coal, thus retarding pore pressure transmission in coal measure strata. Then, the proposed water-based drilling fluid (WBDF) system (4 wt% sodium bentonite + 1.5 wt% sodium carboxymethyl cellulose + 2 wt% lignite resin + 5 wt% potassium chloride + 3 wt%MMH-1 + 0.001 wt% CS-4 + 0.001 wt% CS-1 + 0.001 wt% CS-3) was evaluated based on parameters including rheology, American Petroleum Institute (API) filtration, electrical property, wettability, inhibition capability, reservoir protection characteristics, and anti-pollution performance. It had an API filtration of 7 mL, reservoir damage rate of 10%, moderate and acceptable viscosity, strong inhibition capability to coal measure strata rocks, good tolerance to inorganic pollutants and drilling cuttings, and environmentally friendly properties. It could meet wellbore stability and reservoir protection requirements in the co-exploitation of coal measure gases
Analysis of shale gas accumulation conditions of the Upper Permian in the Lower Yangtze Region
No full textThis paper is the result of oil and gas exploration engineering.
Objective
The dark shale of the Longtan-Dalong Formation is widely developed in the Upper Permian of the Lower Yangtze region, which is the key formation for shale gas exploration in China. At present, the degree of exploration in this area is low, resulting in unclear understanding of shale gas accumulation conditions.
Methods
The geochemical characteristics, reservoir properties, and preservation conditions of the Longtan Formation and Dalong Formation shales were studied to evaluate shale gas exploration potential and optimize favorable exploration areas.
Results
The organic matter abundance of the Longtan Formation and Dalong Formation is high, TOC content is generally greater than 2.0%, the thermal evolution degree is moderate (between 1.3% and 2.5%), the reservoir space is mainly ink bottle and slit-like mesopores, the brittle mineral content is generally higher than 50%, with good hydrocarbon generation material basis and fracturability. The Longtan Formation shale is thick, with good self-sealing ability, and the preservation conditions are generally better than those of the Dalong Formation. Both of them develop various types of fractures, which are the main channels for shale gas escape. In addition, the late magmatic activity is frequent, which has a strong destructive effect on shale gas, and the preservation conditions are the key to shale gas enrichment.
Conclusions
The evaluation of the Upper Permian shale gas in the Lower Yangtze area should be based on sedimentary environment, hydrocarbon generation capacity, and reservoir conditions, taking the shale gas preservation conditions as the key and adopting the principle of finding weak areas in the context of strong tectonic activity. Finally, five favorable areas were divided in the Dalong Formation and Longtan Formation in the Lower Yangtze area
A new bivalve fauna from the Permian-Triassic boundary section of southwestern China
No full textA new marine bivalve fauna from the continuous Upper Permian Longtan Formation to Lower Triassic Yelang Formation of the Zhongzai section in southwestern China is documented. Four bivalve assemblages spanning the Permian–Triassic boundary are recognized and regionally correlated in South China. The bivalve assemblages changed from elements dominated by Palaeozoic types to those dominated by Mesozoic types. Three new species, Claraia zhongzaiensis sp. nov., Claraia sp. nov. 1 and Claraia sp. nov. 2, are described
