Fine Layer Division and Detailed Reservoir Correlation of Ancient Marine Shales in Southern China

Abstract

Although facing many challenges such as the complex geology, terrain and lack of water resources, China has made great breakthroughs in shale gas exploration and development in recent years. The lower part of the Lower Silurian Longmaxi Formation (Longmaxi shale) within the Sichuan Basin, southern China, is the most shale gas-proliferous strata and remains the most vital target for China shale gas production. Deposited in a stable deep-shelf environment, the 20~80m thick Longmaxi marine shale is characterized by high total organic carbon (TOC) content (2.0%~8.4%), high porosity of (3.0%~8.4%), high evolution degree (Ro of 2.0%~3.5%) and ultrahigh pressure (pressure coefficient of 1.3~2.1). Recent drillings have revealed that the Longmaxi shale is not as homogeneous as early understandings predict. In particular, horizontal wells targeted different positions within the Longmaxi shale show great production variances, suggesting high vertical heterogeneity. However, existing vertical layer divisions for the Longmaxi shale are confusing, even contradictory as different nomenclatures and criteria are applied. Using loggings, cores from Changning, Weiyuan and Zhaotong shale gas blocks and outcrop characterizations, a composite layer division scheme is proposed, which divides the 20~80m thick Longmaxi shale, from bottom to top, into four fine layers (1,2,3 and 4). Layer 1 is mainly composed of black carbonaceous shale corresponding to the Persculptogr persculptus biozone (LM1), characterized by high GR values (200~500 API) and the lowest density values (2.1g/cm3~2.5g/cm3). It overlies the Ordovician Wufeng Formation, which is characterized by abrupt lowering GR values and abundant Hirnantia Fauna. Layer 2 is composed of carbonaceous shale and muddy shale with calcareous concretions, it corresponds to the Akidograptus ascensus biozone and Parakidogr acuminatus (LM2~3) and is characterized by gyro-type GR curves (160~270 API) and lower siliceous mineral proportions (~40%) than layer 1 (~50%). Layer 3 is composed by calcareous shale with the highest calcareous mineral abundance (10%~20%), an abrupt GR curve change occurs at the bottom of layer 3, suggesting a sea level fall. Graptolite with most kinds and abundances are developed in layer 3, marking the Cystograptus vesiculosus (LM4). Layer 4 becomes silt-prone with the lowest GR values (120~150API) among the four layers, horizontal beddings are poorly developed. It marks the Coronograptus cyphus (LM5) but graptolites are relatively less abundant in layer 4. Based on this fine layer division, detailed correlation including more than 50 wells across the Sichuan Basin is carried out, demonstrating the Longmaxi reservoir is rather heterogeneous. Regionally, Channing-Shizhu regions have the thickest Longmaxi deposition and the Gongxian-Yongchuan regions have the highest TOC. Vertically, layer 1 has the highest TOC, highest gas content and highest porosity, making it the most favorable target. Such findings could improve the understanding of deep-sea sedimentation during the Lower Silurian and could benefit ancient marine shale gas exploration and development in China, even the world.

AAPG Datapages/Search and Discovery Article #90333©2018 AAPG Middle East Region, Shale Gas Evolution Symposium, Manama, Bahrain, December 11-13, 2018