Dynamic Evolution of Mudstone/Shale Caprocks in the Lower Paleozoic Sequence, Sichuan Basin, SW China

Zhijun Jin, Yusong Yuan, Dongsheng Sun, Quanyou Liu, and Shuanjian Li
Sinopec Petroleum Exploration and Production Research Institute, Beijing, China

The Lower Paleozoic marine carbonate sequences in China have experienced multiphase tectonic events, and developed several sets of source beds and reservoir rocks, with multistage hydrocarbon generation, migration, accumulation, readjustment or destruction. Giant discoveries such as the Tahe - Lunnan oilfields and Jingbian gasfield have revealed the significant hydrocarbon resource potential in the Lower Paleozoic marine carbonate sequences in China. However, the presence of large paleo- oil accumulations in the Majiang, Kaili and Wangcun areas in southern China and the Silurian asphaltic sandstone in the Tarim Basin also indicates pervasive destruction of early oil accumulations in the Lower Paleozoic strata. Therefore, the presence or absence of effective regional hydrocarbon caprocks has played a key role in the petroleum accumulation and preservation in the Lower Paleozoic strata. Static estimation of the present-day sealing capacity of the cap rocks in previous works did not address the preservation effectiveness of the caprocks. How to estimate dynamically the sealing capability of caprocks is one of the fundamental geological problems that have to be resolved in petroleum exploration. In this paper, we proposed a porosity-capillary pressure (φ-Pc) method to estimate the sealing capacity during caprock burial and a permeability-capillary pressure (K-Pc) plus overconsolidation ratio (OCR) method to estimate the caprock sealing capacity during uplifting. Based on 120 capillary pressure - porosity data collected from the previous work, a φ-Pc mathematical model has been established. From the permeability data measured from 12 mudstone/shale samples from different stratigraphic intervals with step loading condition, a permeability-confining pressure (K-P) mathematical model has been constructed. Moreover, capillary pressure - permeability data measured from 141 mudstone or shale cap rocks have been used for establishing a K-Pc mathematical model. Additionally, the OCR of mudstone or shale was calculated from the results of burial and uplifting history reconstructions. These mathematical models are used in a case study in the Sichuan basin, South China. The results indicate that the micro-parameter (Pc) has inherited deficiencies in evaluating sealing capacity of caprocks but the combination of parameter Pc with OCR works very well. The excellent agreement between the calculated data from the mathematic models and the measured formation water data suggests the great utility of our proposed method for dynamic estimation of the sealing capacity of mudstone/ shale caprocks.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013