Why does petroleum in carbonate reservoirs tend to occur at higher temperatures than in sandstones?, by Paul H. Nadeau and Stephen N. Ehrenberg. Previous studies of the temperature distribution of conventionally recoverable oil and gas volumes have shown that most of the Earth’s estimated ultimate recovery occurs at reservoir temperatures of 60 C to 120 C±2 C, termed the “golden zone” for petroleum exploration. Both sandstone and carbonate reservoirs display similar overall conformance to this general pattern, with gradual decrease toward lower temperatures and precipitous dropoff >122 C. This paper compares these two main reservoir lithologies in greater detail in terms of their temperature distributions and volumetric dominance.

Organic pore preservation in marine organic-rich shales of southern China: Insights from pore fluid pressure evolution, by Tao Luo, Xiaowen Guo, Zhiliang He, Zhihui Xiao, Jian-xin Zhao, Tian Dong, Rui Yang, and Ze Tao. Marine organic-rich shales in southern China maintained overpressure conditions caused by hydrocarbon thermal cracking during the deep-burial stage and gas leakage during the period of tectonic uplift, accompanied by organic pore development and preservation. Presently, there is no comprehensive framework for revealing the preservation of organic pores in shale reservoirs. The authors used multiple approaches to quantitatively evaluate organic pores and investigate pore fluid pressure evolution in the Wufeng Formation and Longmaxi Formation shale reservoir in the eastern Sichuan Basin.

Porosity evolution and geometry in Santos Basin Aptian presalt petrofacies, by William da Silveira Freitas, Thisiane Dos Santos, Mariane C. Trombetta, Sabrina Danni Altenhofen, Argos Belmonte Silveira Schrank, Guilherme A. Martinez, Anderson J. Maraschin, Felipe Dalla Vecchia, Amanda Goulart Rodrigues, Luiz Fernando De Ros, and Rosalia Barili. The presalt deposits from offshore southeastern Brazil account for approximately 3/4 of the total hydrocarbon production of the country. The Barra Velha Formation constitutes the main reservoirs of the presalt sag section. The pore systems of these rocks are highly complex, owing to depositional and diagenetic controls. To better understand and characterize the pore systems of the unusual presalt reservoirs, this study aimed to recognize the relationships among their porosity and permeability values and pore types within the context of the evolution and geometry of their pore systems.

Formation, preservation, and release of overpressure in shale gas reservoirs, by Qing He, Tian Dong, Matthew Steele-MacInnis, Zhiliang He, and Dongfeng Hu. Formation and evolution of fluid overpressure is critical to the generation, migration, and accumulation of natural gas. However, how and why fluid overpressure in shales is either preserved or dissipated has received little attention. The authors investigate the generation and preservation of fluid overpressure in different parts of the Wufeng•Longmaxi shales. These rocks represent a good test case to understand preservation versus loss of over-pressure because different drill holes into the same stratigraphic unit variability show conditions of either normal pressure or significant overpressures.