Typical meter-scale lithofacies cycles from the Wolfcamp A in the Delaware and Midland Basins comprise basal carbonate facies overlain by calcareous or siliceous mudrocks. Siliceous mudstones are the most organic-rich facies with high total organic carbon (TOC > 3 wt. %), whereas thin carbonate beds have the lowest organic matter (OM) content among the lithofacies present (TOC < 1 wt. %). To determine if the thin carbonate-rich beds serve as either reservoir storage for migrated oil or act as a seal, three core intervals of meter-scale lithofacies cycles in the Wolfcamp A were selected for sampling and were analyzed using x-ray diffraction analysis, TOC, programmed pyrolysis analysis, and residual gas analysis from rock crushing.Oil saturation index (OSI) (the amount of free oil normalized by TOC; OSI = S1 × 100/TOC) is used as an indicator of oil enrichment or depletion in the reservoir, where S1 is volatile oil in programmed pyrolysis (temperature = 300°C). Both TOC-lean carbonate and TOC-rich mudstone lithofacies have high OSI in these meter-scale cycles (average OSI is 124.5 mg HC/g TOC for carbonate beds), indicating that migrated oil is present. Residual gas analyses show lower dryness values (C1/C1–5) and higher oil indicator values (100 × C4+5/C1–5) in TOC-lean carbonate beds compared to the TOC-rich mudstones, likely indicating a cumulative oil and gas charging effect through source rock maturation. Oil and gas generated at different stages of thermal maturation were partially expelled from OM-rich siliceous/calcareous mudstones into adjacent OM-lean carbonate beds. This study shows oil expulsion from source to adjacent carbonate beds is a key factor in variations of oil saturation in the Wolfcamp A.

中文翻译：

---

米尺度岩相循环和对油饱和度变化的控制，Wolfcamp A、特拉华州和米德兰盆地

特拉华州和米德兰盆地 Wolfcamp A 的典型米级岩相旋回包括覆盖有钙质或硅质泥岩的基底碳酸盐岩相。硅质泥岩是有机质含量最高的相，具有高总有机碳（TOC > 3 wt. %），而薄碳酸盐岩层的有机质（OM）含量在现有岩相中最低（TOC < 1 wt. %）。为了确定富含碳酸盐的薄层是作为运移油的储层存储还是作为密封，选择了 Wolfcamp A 中米级岩相循环的三个核心层段进行采样，并使用 X 射线衍射分析进行分析， TOC、程序热解分析和岩石破碎残余气分析。OSI = S1 × 100/TOC) 用作储层中石油富集或枯竭的指标，其中 S1 是程序热解中的挥发油（温度 = 300°C）。在这些米级旋回中，贫 TOC 碳酸盐岩和富含 TOC 的泥岩岩相都具有高 OSI（碳酸盐岩床的平均 OSI 为 124.5 mg HC/g TOC），表明存在运移的油。残余气分析表明，与富含 TOC 的泥岩相比，贫 TOC 的碳酸盐岩层具有较低的干度值 (C1/C1-5) 和较高的油指标值 (100 × C4+5/C1-5)，这可能表明累积的油和烃源岩成熟的充注作用。在热成熟的不同阶段产生的油气被部分地从富含有机质的硅质/钙质泥岩中排出到相邻的贫有机质碳酸盐岩层中。