Calculation and inducement of lacuna in the Mid-Permian Maokou Fm of Sichuan Basin

Abstract

Erosion of carbonate strata not only can reflect the karst landform, but also can reveal the characteristics of carbonate karst reservoirs indirectly. In Sichuan Basin, the quantity and root causes of lacuna in the top strata of the Maokou Fm are unclear. In this paper, the features of such stratigraphic lacuna were analyzed qualitatively. Then, in accordance with the spectral transformation of GR logs and the characteristics of cycle in the Maokou Fm penetrated by four typical wells, the Milankovich cycles of the logs were identified. Then they were used for quantitative calculation of erosion quantity and for highlighting the root causes for the stratigraphic lacuna. According to the research results, the Mao-4 Member is residual in the Yibin–Ya'an–Jiangyou area and the Shizhu area, but missing in the other parts of Sichuan Basin with a stratigraphic lacuna intensity gradually increasing from the southern part to the central part and to the northern parts of the basin. Besides, the stratigraphic lacuna of the Maokou Fm varies from 0 to 200 m – for example, 0–60 m in the southwestern and northeastern parts of the basin, and 140–200 m in the southern, central and northern parts of the basin. Moreover, formation denudation occurred at the end of the Maokou Period was induced predominantly by erosion generated as the sea level fell during the glacial age, especially in the northern part of the basin. Also, the Mid-Permian karst landform inherited the sedimentary features of being high in the southwest and low in the northeast. In the areas from the southwestern to central and to the northern Sichuan Basin, the karst landform transits from erosion highlands to karst upper and lower slopes, coinciding well with the sedimentary features of the Wujiaping Period. In conclusion, the Milankovich cycles can be used to calculate the stratigraphic erosion in carbonate formations efficiently and accurately. The technique can be extended to the restoration of stratigraphic erosion in other marine basins. Keywords: Sichuan basin, Mid-Permian, Maokou Fm, Milankovich cycle, Stratigraphic erosion, Quantitative calculation, Glacial age, Genesis discussion, Karst landform