Jinliang Huang

Shale gas generation and potential of the Lower Cambrian Qiongzhusi Formation in the Southern Sichuan Basin, China

Abstract According to the latest drilling data, the geologic survey of surrounding outcrops and the laboratory test results of rock samples, this paper studies the regional distribution, geochemical and reservoir characteristics and gas concentration of organic-rich shales to reveal the shale gas generation and potential of the Lower Cambrian Qiongzhusi Formation of the Southern Sichuan Basin. The study area features high organic content (with TOC between 0.55% and 25.7%, average of more than 2%), great net shale thickness (of 60–300 m), high brittle mineral content (over 40%), abundant micro- and nanometer-sized pores and micro-fractures, and high gas content (0.27–6.02 m3/t, average of 1.90 m3/t), which is in favor of the generation and accumulation of shale gas. The Lower Cambrian Qiongzhusi Formation is one of the most favorable strata which has frequent gas show and industrialization breakthrough during drilling, and the Weiyuan and Xunyong-Junlian areas are two most realistic and favorable targets for shale gas exploration and development.

Shale gas in China: Characteristics, challenges and prospects (II)

Abstract This paper mainly discusses the industrialization progress, “sweet spot” evaluation criterion, E&P technologies, success experiences, challenges and prospects of Chinas shale gas. Based on the geologic and engineering parameters of the Fuling, Changning and Weiyuan shale gas fields in the Sichuan Basin, this paper points out that Chinas shale gas has its particularity. The discoveries of super-giant marine shale gas fields with high evolution degree (R o =2.0%−3.5%) and ultrahigh pressure (pressure coefficient=1.3−2.1) in southern China is of important scientific significance and practical value to ancient marine shale gas exploration and development to China and even the world. Its proposed that shale gas “sweet spots” must be characterized by high gas content, excellent frackability and good economy etc. The key indicators to determine the shale gas enrichment interval and trajectory of horizontal wells include “four highs”, that is high TOC (>3.0%), high porosity (>3.0%), high gas content (>3.0 m 3 /t) and high formation pressure (pressure coefficient>1.3), and “two well-developed” (well-developed beddings and well-developed micro-fractures). Its suggested that horizontal well laneway be designed in the middle of high pressure compartment between the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation. The mode of forming “artificial shale gas reservoir” by “fracturing micro-reservoir group” is proposed and the mechanisms of “closing-in after fracturing, limiting production through pressure control” are revealed. Several key technologies (such as three-dimensional seismic survey and micro-seismic monitoring of fracturing, horizontal wells, “factory-like” or industrialized production mode, etc.) were formed. Some successful experiences (such as “sweet spot” selection, horizontal well laneway control, horizontal length optimization and “factory-like” production mode, etc.) were obtained. The four main challenges to realize large-scale production of shale gas in China include uncertainty of shale gas resources, breakthroughs in key technologies and equipment of shale gas exploration and development below 3 500 m, lower cost of production, as well as water resources and environment protection. It is predicted that the recoverable resources of the Lower Paleozoic marine shale gas in southern China are approximately 8.8×10 12 m 3 , among which the recoverable resources in the Sichuan Basin are 4.5×10 12 m 3 in the favorable area of 4.0×10 4 km 2 . The productivity of (200−300)×10 8 m 3 /a is predicted to be realized by 2020 when the integrated revolution of “theory, technology, production and cost” is realized in Chinese shale gas exploration and development. It is expected in the future to be built “Southwest Daqing Oilfield (Gas Daqing)” in Sichuan Basin with conventional and unconventional natural gas production.

Quantitative characterization of reservoir space in the Lower Silurian Longmaxi Shale, southern Sichuan, China

Based on the drilling data of the Upper Ordovician Wufeng Shale and the Lower Silurian Longmaxi Shale in southern Sichuan Basin, the construction of matrix pores and the development condition of fractures in a marine organic-rich shale are quantitatively evaluated through the establishment of the reservoir petrophysical models and porosity mathematical models. Our studies show that there are four major characteristics of the Longmaxi Shale confirmed by the quantitative characterization: (1) the pore volume of per unit mass is the highest in organic matter, followed in clay minerals, finally in brittle minerals; (2) the porosity of the effective shale reservoir is moderate and equal to that of the Barnett Shale, and the main parts of the shale reservoir spaces are interlayer pores of clay minerals and organic pores; (3) the porosity of the organic-rich shale is closely related to TOC and brittle mineral/clay mineral ratio, and mainly increases with TOC and clay mineral content; (4) fractures are developed in this black shale, and are mainly micro ones and medium-large ones. In the Longmaxi Shale, the fracture density increases from top to bottom, reflecting the characteristics with high brittle mineral content, high Young’s modulus, low Poisson’s ratio and high brittleness at its bottom.

Geological difference and its significance of marine shale gases in South China

Abstract Organic-rich marine shales are developed in both the Cambrian Qiongzhusi Formation and the Ordovician Wufeng Formation–Silurian Longmaxi Formation in South China, but are different in the drilling results of shale gas exploration. Comparing the differences in shale gas formation conditions between Qiongzhusi and Wufeng-Longmaxi has practical and theoretical significance. This study reveals: (1) in the Sichuan Basin, the Wufeng-Longmaxi Formation has slightly higher TOC than the Qiongzhusi Formation, whereas Qiongzhusi Formation has some local high TOC areas outside of the Sichuan Basin; (2) the Qiongzhusi Formation has much higher thermal evolution degree than the Wufeng-Longmaxi Formation; (3) with undeveloped organic pores, the Qiongzhusi Formation has a 1/3 to 1/2 porosity of the Wufeng-Longmaxi Formation; (4) Qiongzhusi shale has a lower gas content, only 1/2 of that in Wufeng-Longmaxi shale; (5) the Qiongzhusi Formation is mainly composed of siliceous shale and the silica is hot water origin, whereas the Wufeng-Longmaxi Formation consists mainly of calcareous siliceous shale and the silica is biogenic origin; (6) the Wufeng-Longmaxi Formation has overpressure, while the Qiongzhusi Formation is normal in pressure. The reasons for the differences are: (1) different sedimentary environments affect TOC and shale thickness; (2) the Qiongzhusi Formation is over-mature, which caused depletion of hydrocarbon generation, organic carbonization, porosity reduction, and gas content drop; (3) the bad roof and floor conditions of the Qiongzhusi Formation are not good for shale gas preservation; (4) Wufeng-Longmaxi Formation is located in the slope and syncline accompanied with overpressure, and is in favor of preservation and high production of shale gas; (5) the uranium content in the Qiongzhusi Formation is twice that of the Wufeng-Longmaxi Formation, which is the main reason of its higher thermal evolution degree. It is concluded that shale gas enrichment in the marine shale in South China requires favorable geological conditions: organic-rich intervals, moderate thermal evolution, rich organic pores, high gas content, good roof and floor preservation conditions, and moderate burial depth. The Wufeng-Longmaxi Formation has better shale gas enrichment conditions and higher resource potential, whereas the Qiongzhusi Formation has poorer shale gas accumulation conditions and limited favorable areas.

Methods for Shale Gas Play Assessment: A Comparison between Silurian Longmaxi Shale and Mississippian Barnett Shale

Based on field work, organic geochemical analyses and experimental testing, a six-property assessment method for shale gas is proposed. These six properties include organic matter properies, lithofacies, petrophysical properties, gas content, brittleness and local stress field. Due to the features of continuous distribution over a large area and low resource abundance in shale plays, a sweet spot should have these following properties: (a) TOC>2%; (b) brittle minerals content (>40%) and clay minerals (<30%); (c) Ro (>1.1%); (d) porosity (>2%) and permeability (>0.000 1 mD), and (e) effective thickness (30–50 m). Applying these criteria in the Sichuan Basin, the Silurian Longmaxi shale consists of four prospecting sweet spots, including blocks of Changning, Weiyuan, Zhaotong and Fushun-Yongchuan. Although these four blocks have some similarities, different features were usually observed. After comprehensive analyses using the six-property assessment method, the Fushun-Yongan Block ranks the most favorable sweet spot, followed by the Weiyuan Block. For the other two blocks, the Changning Block is better than the Zhaotong Block. By comparing with the Mississippian Barnett shale, characteristics that are crucial for a high-yielding in the Sichuan Basin include a high content of organic matter (TOC>2.5%), a moderate thermal maturity (Ro=0.4%–2%), a high content of brittle minerals (quartz: 30%–45%), a high gas content (>2.5 m3·t−1), and types I and II1 kerogen.

Guanyinqiao Member lithofacies of the Upper Ordovician Wufeng Formation around the Sichuan Basin and the significance to shale gas plays, SW China

Abstract The stratigraphic distribution and lithofacies features of the Guanyinqiao Member of the Upper Ordovician Wufeng Formation around the Sichuan Basin were studied based on the data of drilling, outcrops, rock minerals, geochemistry and logging. The stratigraphic distribution of the Guanyinqiao Member is generally dominated by sea-level variation. This formation is in conformable contact with the overlying and underlying formations inside and locally outside the basin and is locally lost outside the basin. The Guanyinqiao Member is usually composed of deep-water siliceous shale and calcareous and siliceous shale facies, similar to those overlying and underlying lithologies inside the basin; and composed of shallow marl facies or lost, greatly different from the surrounding lithologies in the conformable area outside the basin. The study confirmed that the lithofacies of the Guanyinqiao Member is of great significance to the identification of the reservoir properties of the Wufeng and Longmaxi formations shale gas and to the selection of shale gas. The exploration and development of shale gas is promising in the depressions in the south, east and northeast of the Sichuan Basin, where good reservoirs are developed. The zone where the Guanyinqiao Member is lost has no “sweet spot” formation and is risky in exploring shale gas. The reservoir quality and exploration prospect of the Guanyinqiao Member marl zones are between the two zones mentioned above.

Geochemical and reservoir characteristics of the Upper Triassic continental shale in the Sichuan Basin, China

The drilling data and test results of a large number of shale samples collected from Upper Triassic Xujiahe Formation, in the Sichuan Basin were used to find out the distribution characteristics, geochemical characteristics, reservoir characteristics and gas-bearing property of the continental shale in Xujiahe Formation, to figure out its resource prospects. The shale reservoir of the Xujiahe Formation was high in organic matter content (with a TOC content of 0.06% to 14.8%, 2.36% on average), with an organic matter of type III kerogen, Ro% of 0.8% to 3.0% (1.62% on average). In the high maturity stage of large quantity gas generation, it is rich in brittle mineral (>40%), and high in clay mineral content (average of 49.5%) which is mainly composed of mixed illite/smectite layer and illite. The shale is 1%–3% in porosity (2.73% on average). The pores are mainly nanoscale matrix ones (2 to 50 nm in size), and organic pores in local areas. The gas content of Xujiahe shale was measured at 3.01 m3/t, on average. This study suggests that Xujiahe shale has some gas resource potential; but, there are still a number of challenges and difficulties which need to be resolved in its exploration and development in the future.

Geological Conditions and Prospect Forecast of Shale Gas Formation in Qiangtang Basin, Qinghai-Tibet Plateau

: The presence of shale gas has been confirmed in almost every marine shale distribution area in North America. Formation conditions of shale gas in China are the most favorable for marine, organic-rich shale as well. But there has been little research focusing on shale gas in Qiangtang Basin, Qinghai-Tibet Plateau, where a lot of Mesozoic marine shale formations developed. Based on the survey results of petroleum geology and comprehensive test analysis data for Qinghai-Tibet Plateau, for the first time, this paper discusses characteristics of sedimentary development, thickness distribution, geochemistry, reservoir and burial depth of organic-rich shale, and geological conditions for shale gas formation in Qiangtang Basin. There are four sets of marine shale strata in Qiangtang Basin including Upper Triassic Xiaochaka Formation (T3x), Middle Jurassic Buqu Formation (J2b), Xiali Formation (J2x) and Upper Jurassic Suowa Formation (J3s), the sedimentary types of which are mainly bathyal-basin facies, open platform-platform margin slope facies, lagoon and tidal-flat facies, as well as delta facies. By comparing it with the indicators of gas shale in the main U.S. basins, it was found that the four marine shale formations in Qiangtang Basin constitute a multi-layer distribution of organic-rich shale, featuring a high degree of thickness and low abundance of organic matter, high thermal evolution maturity, many kinds of brittle minerals, an equivalent content of quartz and clay minerals, a high content of feldspar and low porosity, which provide basic conditions for an accumulation of shale gas resources. Xiaochaka Formation shale is widely distributed, with big thickness and the best gas generating indicators. It is the main gas source layer. Xiali Formation shale is of intermediate thickness and coverage area, with relatively good gas generating indicators and moderate gas formation potential. Buqu Formation shale and Suowa Formation shale are of relatively large thickness, and covering a small area, with poor gas generating indicators, and limited gas formation potential. The shale gas geological resources and technically recoverable resources were estimated by using geologic analogy method, and the prospective areas and potentially favorable areas for Mesozoic marine shale gas in Qiangtang Basin are forecast and analyzed. It is relatively favorable in a tectonic setting and indication of oil and gas, shale maturity, sedimentary thickness and gypsum-salt beds, and in terms of mineral association for shale gas accumulation. But the challenge lies in overcoming the harsh natural conditions which contributes to great difficulties in ground engineering and exploration, and high exploration costs.

Shale Gas in China: Reality and Dream

China extracted shale gas of 23, 500∼24, 600 m3/d from the Lower Cambrian Qiongzhusi Shale Formation in Weiyuan gas field of Sichuan Basin in 1966. This shale interval, however, was considered less prospective because the Sinian Doushantuo Formation dolomite reservoir was defined as the target layer for drilling at that time. Forty years later, this black shale interval became attractive as triggered by the “Shale Gas Evolution” in North America. In 2005, by learning from shale gas mode of the United States, PetroChina Research Institute of Petroleum Exploration and Development (RIPED) reviewed the existing well data of the region where the Weiyuan and Longchang gas fields and the Changning structure were located within the Southern Sichuan Basin. In order to re-examine the gas shows and geochemical property and to evaluate the gas potential of shale strata within the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation and the Lower Cambrian Qiongzhusi Formation. On that basis, for the entire Sichuan Basin and adjoining regions, PetroChina has carried out a geologic survey on organic-rich shales, drilled shallow shale gas appraisal wells, evaluated the shale gas resource potential, and defined some favorable targets for shale gas exploration and development by 2008. W201, the first shale gas appraisal well of China, drilled by PetroChina in 2009, initially produced 17, 000 m3/day from the Wufeng-Longmaxi Formation shale and 10, 800 m3/day from the Qiongzhusi Formation shale, marking the commencement of shale gas exploration and development activities in China. Thus far, China has received shale gas flow from marine, transitional-limnetic coaly and lacustrine shales in many regions, such as Sichuan, Chongqing, Guizhou, etc. The commercialization of marine shale gas has been achieved in Sichuan and Chongqing totaling 0.2 billion cubic meters of shale gas production in 2013. Through an overall review on the progress of shale gas exploration and development in China, we concluded that the geological conditions for shale gas in China are extremely complex. First, the most sedimentary basins in China belong to superimposed basins, all of them have experienced the depositional evolution from marine to terrestrial facies, and three types of shales were well developed including marine, transitional, and terrestrial organic-rich shales. Second, the conditions of shale gas formation and accumulation in China are favorable and shale gas resource has a great potential for further development in China. Third, like North America, the marine organic-rich shales of China have the best and most realistic shale gas exploration and development prospects. But it faces technical, environmental, regulatory, infrastructural and other challenges that are likely to delay rapid shale gas development. Fourth, until now, although the exploration and exploitation of shale gas in China are relatively low, very good signs have been emerged to prove that China has good prospects in the future.