Porosity of Overmature Marine Shale and Free Gas Volumes: Key Factor to the Successful Commercial Development of Shale Gas in the South China

Feiyu Wang^1,2, Jing Guan¹, Weiping Feng¹, Linyan Bao¹, and Zhiyong He^3
1State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China
²College of Geoscience, China University of Petroleum, Beijing, China
³Zetaware Inc., Sugar Land, Texas, USA

Our understanding of shale plays have evolved very quickly in the last 10 years since we started looking into it. In the beginning, the focus was on adsorption as the mechanism for gas storage in source rocks. Then we realized that most of the gas was actually free gas, just like in a conventional reservoir. What makes it unconventional is that it requires fracking to produce. The free gas in porosity is most important for shale gas plays, as production of adsorbed gas may be limited in shale compared to coal bed methane. At the pressure of the typical depth for shale gas production, adsorption is not a strong function of pressure and desorption may be difficult according to the adsorbed gas curve as a function of pressure. The free gas in porosity is the key factor for shale gas development, free gas volumes is controlled by porosity and gas saturation.

Lower Silurian Longmaxi Formation shale and Lower Cambrian Qiongzhusi Formation shale are two most important shale plays in the in the South China, Petrologic and geochemical parameters suggest that maturity reach to 2.0-3.0%VRo and 2.5-5.5%VRo respectively, which raises critical questions regarding metagenetic gas generation and reservoir quality of the shales. Over maturity and deep burial diagenesis implicates mechanical and chemical compaction, cementation, and alteration, which may enhance or diminish the reservoir potential. 86 samples were selected from overmature Cambrian and Silurian marine shales from Sichuan Basin and marginal areas to analysis porosity with He-Hg method and FIB-SEM with ion milling and high resolution imaging analysis, these data indicate that porosity of overmature marine shale range 1.5% to 10%, the organic-rich (TOC 2 ~ 8%) lower Silurian Longmaxi Formation shale display the higher porosity with 5 ~ 10%, which indicate the organic-rich shales display higher porosity and gas saturation. Comparison of porosity data of three marine shale plays with three different maturity stage (1.5-1.8%, 2.0-2.3%, 2.5-3.1%) manifest porosity decrease significantly with increasing maturity in the overmature stage (Fig1). when VRo is more than 2.5%, which diminish free gas volume in shale.

The typical porosity value of organic matter rich shales from Lower Silurian Longmaxi Formation (2.0-2.3%VRo) is about 6% with gas saturation 80-90%, in which free gas volume range 6.0-6.8 m³/t rock at the depth 3000m. With this correspondence, the typical porosity value of organic matter rich shale from Lower Cambrian Qiongzhusi Formation (2.8-3.1%VRo) is about 2.5% with gas saturation 50-60%, in which free gas volume range 1.2-2.3 m³/t rock at the depth 3000m. The difference in porosity of shale result in the significant variation of free gas volume, which will be help understand the shale gas initial production contrast of Lower Silurian Longmaxi Formation shale play and Lower Cambrian Qiongzhusi Formation shale play

Engelder and some others have suggested that there is a thermal maturity upper limit (about 3.2%VRo) for commercial development of shale gas in the Marcellus shale play of Pennsylvania, that he termed the “line of death”. The completed shale gas wells in the South China indicate that highly productive wells are limited to Lower Silurian Longmaxi Formation shale play, and most of wells of Lower Cambrian Qiongzhusi Formation shale play do not appear to have produced economically. Thus, the possibility of an upper limit of thermal maturity is a critical question for successful commercial development of shale gas in the South China. According to the map-based database analysis of Marcellus play, Fayetteville play, Woodford play, and data of completed shale gas wells in the South China, We consider that commercial development value of shale gas are reduced or deteriorated when the maturity is more than 3.0 ~ 3.5%, the geological cause lie in diminishing of free gas volume with the gas porosity were decreased.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013