四川盆地中二叠统茅口组地层缺失量计算及成因探讨

碳酸盐岩地层剥蚀量不仅可以反映岩溶地貌,而且还能间接反映碳酸盐岩岩溶储层的发育特征。针对四川盆地中二叠统茅口组顶部地层缺失量及成因不清的问题,在定性分析地层缺失特征的基础上,根据4口典型井茅口组的自然伽马测井曲线的频谱变换、旋回特征分析,识别出曲线中的米兰科维奇旋回信息,进而采用米兰科维奇旋回法定量计算了该地层的剥蚀量并探讨其缺失成因。结果表明:(1)茅四段仅在宜宾—雅安—江油地区和石柱地区残存,在其余地区则普遍缺失,并且表现为由川南—川中—川北地区地层缺失强度逐渐加大;(2)茅口组地层缺失量介于0~200 m,其中川西南、川东北地区缺失厚度介于0~60 m,川南、川中、川北等地区地层缺失厚度介于140~200 m;(3)茅口期末的地层剥蚀是由冰期海平面下降侵蚀所致,川北地区海平面下降幅度较大;(4)中二叠统岩溶地貌继承了西南高东北低的沉积特征,岩溶地貌从川西南—川中—川北地区,由侵蚀高地逐渐过渡为岩溶上斜坡和岩溶下斜坡,与吴家坪期西南高东北低的沉积特征一致。结论认为,采用米兰科维奇旋回法计算碳酸盐岩地层剥蚀量结果可靠、方法有效,可用于其他盆地海相地层缺失量的恢复研究。

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

Erosion of carbonate strata not only can reflect the karst landform, but also can reveal the characteristics of carbonate karst reservoirs indirectly. In the 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 the 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 period, 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.