Reservoir Space Characterization & Property of Terrestrial Shale in China
Rukai Zhu1,2, Songtao Wu1,2, Bin Bai1,2, Jingwei Cui1,2, Tuo Wang1, and Tingting Li1
1Research Institute of Petroleum Exploration & Development, PetroChina, Beijing, China
2CNPC Key Laboratory of Oil and Gas Reservoirs, Beijing, China
Unconventional oil & gas with great resource potential is becoming the important exploration target globally. The detailed research on unconventional oil & gas reservoirs is critical for the exploration and development. The micro-structure, i.e., pore-throat size, shape & connectivity, micro-porosity evolution and oil & gas occurrence have become the important aspects in unconventional reservoirs evaluation.
Abundant work on marine shale porosity and reservoir potential has been carried out in North America. This paper is focused on the widespread terrestrial shale system in China and the samples include the Chang 7 shale in Ordos Basin, Lucaogou Fm. in Junggar Basin, Jurassic shale in Sichuan Basin and Qingshankou Fm. in Songliao Basin. SEM, Nano-CT and FIB-SEM are used to characterize the storage space and to reconstruct the 3D porosity model. The porosity size and distribution are analyzed based on image analysis, mercury injection and gas adsorption data. Moreover, the porosity evolution of terrestrial shale with low maturity is studied based on the reservoir diagenetic modeling system which is designed by CNPC Key Laboratory of Oil & Gas Reservoirs.
The lacustrine shale in China has the potential to produce industrial oil and gas. The characteristics are as follows:
(1) Shale are deposited in semi-deep/deep lacustrines and Type I and IIA dominate the kerogen. Ro is 0.7%~1.2%, TOC is 1.4% ~ 25.6%, S1 is 1.15 ~ 21.6mg/g rock, and Chloroform bitumen A is 0.25%~1.5%. The percentage of detained hydrocarbon (HC) in total generated HC is 20%~50%, indicating great resource potential in shale system.
(2) The brittleness index of shale is relatively high. XRD data suggest that the content of brittle mineral including quartz, feldspar, calcite and dolomite is 41% in average and that of clay mineral is less than 50%. Taking the Lucaogou Fm. in Junggar Basin for example, the content of clay mineral is less than 30%, which is good news for the further hydro-fracturing based on the experiences from unconventional oil & gas projects in North America.
(3) Typical 3-element structure is developed in terrestrial shale. Brittle minerals, clay minerals and organic matter are inter-bedded from bottom to top. Nano-pores and micro-fractures diametered in 50~300nm dominate the storage space. Three types of pores include inter-particle pores, intra-particle pores and OM pores. The percentage of inter-clay mineral pores and OM pores is 50% and 30% respectively. Moreover, there are some isolated pores at micrometer scale.
(4) The pore-throat system in terrestrial shale is connected as a whole, and the connected frame is composed by inter-particle pores, OM pores and bedding fractures. The connectivity index is over 60%, although some isolated pores developed.
(5) The content of pyrites is positively correlated to TOC. Environment SEM data indicate that abundant HC is located around pyrites. The liquid HC is absorbed in the surface and inner OM. In inter-particle pores of pyrites, clay minerals and brittle minerals, the liquid HC can be in absorbed state or free state. Liquid HC is free in the bedding fractures.
(6) Reservoir diagenetic modeling for terrestrial shale with low maturity indicates that the porosity in shale is controlled by maturity, clay mineral content and compaction. The evolution of pore volume per kilogram of macro-pores, meso-pores and micro-pores is different. The pore volume per kilogram of macro-pores increases firstly and decreases later with increasing temperature & pressure, while that of meso-pores and micro-pores suggesting the opposite tendency. The content of HC in shale shows the same tendency to macro-pores and reaches its maximum value of 150 mg/g TOC at 350°C. The evolution of OM pores is controlled by the organic maceral and maturity.
AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013