Shi Siyu, Ding Wenlong, Zhao Gang, Xiao Zikang, Li Yuntao, Jiao Baocheng
School of Energy Resources, China University of Geosciences, Beijing 100083, P. R. China.
SINOPEC Petroleum Exploration and Production Research Institute, Beijing 100083, P. R. China.
ACS Omega. 2021 Mar 10;6(11):7558-7575. doi: 10.1021/acsomega.0c05989. eCollection 2021 Mar 23.
The Middle Permian Maokou Formation in the southeastern Sichuan Basin is a typical carbonate karst reservoir. At the end of the Middle Permian, a short-term tectonic uplift (Tungwu movement) occurred in the upper Yangtze region, causing the formation of dissolved fissures and holes. To determine the location of the high-quality reservoir, this paper calculated the eroded thickness using the Milankovitch theory. Based on the gamma logging data of the six wells in the southeastern Sichuan Basin, the dominant frequency and the astronomical time scale were evaluated via frequency spectrum analysis, continuous wavelet transform, and empirical mode decomposition. In addition, we analyzed the relationship between Fischer curve characteristics and the variation of lithology. Last, four methods were used to calculate the eroded thickness, and the rationality was analyzed. Consequently, we identified four levels of Milankovitch cycles, i.e., middle eccentricity (e), short eccentricity (e), long obliquity (o), and short obliquity (o). Also, the Fischer curves of the six wells were divided into two forms related to local structural uplift. The residual strata of the Maokou Formation comprised three complete third-order cycles, and the boundaries were the 15th, 34th, and 54th e cycles. The deposition rate of bioclastic limestone was the lowest (2.12-5.36 cm/ka with an average of 3.30 cm/ka), whereas the deposition rate of argillaceous limestone was the largest (2.27-5.25 cm/ka with an average of 4.09 cm/ka). Among the four methods, the missing formation deposition rate method exhibited the most precise calculation results, while that of the seismic data method was relatively low. Generally, the eroded thickness of the Maokou Formation in southeastern Sichuan was in the range of 0-140 m, i.e., the eroded thickness in the west and south of X14 was relatively large (>100 m), while the area north of LS1 experienced the weakest denudation (eroded thickness < 40 m).
四川盆地东南部中二叠统茅口组是典型的碳酸盐岩岩溶储层。中二叠世末期，上扬子地区发生短期构造隆升（东吴运动），导致溶蚀裂隙和孔洞形成。为确定优质储层位置，本文利用米兰科维奇理论计算了剥蚀厚度。基于四川盆地东南部6口井的伽马测井资料，通过频谱分析、连续小波变换和经验模态分解，评估了主频和天文时间尺度。此外，分析了费舍尔曲线特征与岩性变化之间的关系。最后，采用4种方法计算剥蚀厚度，并分析其合理性。结果识别出米兰科维奇旋回的4个级别，即中偏心率（e）、短偏心率（e）、长倾角（o）和短倾角（o）。6口井的费舍尔曲线分为与局部构造隆升相关的两种形态。茅口组残余地层包含3个完整的三级旋回，边界为第15、34和54个e旋回。生物碎屑灰岩沉积速率最低（2.12 - 5.36 cm/ka，平均3.30 cm/ka），而泥质灰岩沉积速率最大（2.27 - 5.25 cm/ka，平均4.09 cm/ka）。4种方法中，缺失地层沉积速率法计算结果最精确，地震资料法计算结果相对偏低。总体上，四川盆地东南部茅口组剥蚀厚度在0 - 140 m范围内，即X14以西和以南地区剥蚀厚度较大（>100 m），而LS1以北地区剥蚀作用最弱（剥蚀厚度<40 m）。
