Shale Gas Formation and Occurrence in China: An Overview of the Current Status and Future Potential

Abstract

Shale gas is one of the most promising unconventional resources both in China and abroad. It is known as a form of self‐contained source‐reservoir system with large and continuous dimensions. Through years of considerable exploration efforts, China has identified three large shale gas fields in the Fuling, Changning and Weiyuan areas of the Sichuan Basin, and has announced more than 540 billion m3 of proven shale gas reserves in marine shale systems. The geological theories for shale gas development have progressed rapidly in China as well. For example, the new depositional patterns have been introduced for deciphering the paleogeography and sedimentary systems of the Wufeng shale and Longmaxi shale in the Sichuan Basin. The shale gas storage mechanism has been widely accepted as differing from conventional natural gas in that it is adsorbed on organic matter or a mineral surface or occurs as free gas trapped in pores and fractures of the shale. Significant advances in the techniques of microstructural characterization have provided new insights on how gas molecules are stored in micro‐and nano‐scale porous shales. Furthermore, newly‐developed concepts and practices in the petroleum industry, such as hydraulic fracturing, microseismic monitoring and multiwell horizontal drilling, have made the production of this unevenly distributed but promising unconventional natural gas a reality. China has 10–36 trillion m3 of promising shale gas among the world's whole predicted technically recoverable reserves of 206.6 trillion m3. China is on the way to achieving its goal of an annual yield of 30–50 billion m3 by launching more trials within shale gas projects.

Comparative Porosity and Pore Structure Assessment in Shales: Measurement Techniques, Influencing Factors and Implications for Reservoir Characterization

“…Under extreme circumstances, for shales containing very high clay contents and thus high CBW, the most of pore spaces could be nearly fully-occupied by the volume of CBW [8,54] that would influence the petrophysical properties in shales [27,[55][56][57]. As helium porosimetry is able to obtain effective porosity by covering a wider pore size range (i.e., 0.1 nm-100 µm) than MICP (i.e., 3.6 nm-100 µm) ( Figure 10) [34], the CBW is calculated by subtracting the helium porosity (i.e., effective effective) from NMR porosity (i.e., total porosity). Figure 11 cross-plots the calculated CBW versus the clay content in both Carynginia and Monterey shales.…”

“…Moreover, the equation is most likely to fit into the formation with the brine salinity of 20,000-30,000 ppm that matches with our studied formations. The multi-scaled methods for pore characterization in shales Modified from other studies [32,34,58,59]). Apart from the influencing factors associated with clays, the compatibility of the penetrated working fluid molecules with shale nanopore structure also causes the interpretation inconsistencies.…”

“…Fluid penetration methods, i.e., low-pressure (<18.4 psi) CO 2 gas adsorption (LP-CO 2 -GA), low-pressure N 2 gas adsorption (LP-N 2 -GA), mercury intrusion capillary pressure (MICP) and nuclear magnetic resonance (NMR), enable a wide range of pore structure detection and have been universally utilized in shale research studies [23][24][25][26][27][28][29][30][31]. However, MICP displays destructive disadvantages and is not approachable to the pore throat sizes smaller than 3.6 nm [3,11,32,33], merely inter-communicated pores are available for detection [34]. Helium expansion is attainable to the connected pore space corresponding to effective porosity, while the acquisition of PSD is not available [7].…”

“…LP-CO 2 -GA coupled with LP-N 2 -GA is approachable to the pore sizes ranging from 0.35 nm to 200 nm [35]. However, only interconnected pores are accessible [34], and the results are sensitive to measurement procedures and highly dependent on the sample pre-treatment such as the dewatering/ outgassing temperatures and the size of the smashed shale fragments [36][37][38]. NMR, which is acknowledged as a non-destructive technique, is adaptable for measuring the total porosity and PSD in shales [4,27,[39][40][41].…”

A Review of Formation Mechanism Study on Reservoirs with Tilted Oil-water Contacts

“…The neotectonics movement has great influence on the late oil and gas accumulation of the oil and gas basins in China, especially in the unconventional reservoirs such as the titled oil-water contacts reservoir, tight oil and shale gas (Li Xinzi et al, 2013;Pan Yanan et al, 2015;Zou Caineng et al, 2016;Ran Bo et al, 2016;Fan Jianming et al, 2016). In China, the oil and gas basins affected by the neotectonics movement are more developed, such as the Bohai low-uplift area, Bohai Bay continental faulted depression area, Daqing Changyuan belt, western foreland thrust belt, western structural tectonic uplift area, Tarim Craton Paleoproterozoic, East Sichuan fault fold belt, biogas area of Qaidam basin (Jia Chengzao et al, 2006), including the Hudson oil field, Sa er tu, Pu tao hua and Gao tai zi oilfields in Daqing, and other oilfields with large titled oil-water contact.…”