Porosity refers to the ratio of the pore volume in a rock (or the volume of space not filled with solid matter) to the total volume of the rock. It plays a critical role in understanding reservoir properties, especially in oil and gas exploration. The study of porosity differs significantly between continental sequence stratigraphy and passive continental margin marine sequence stratigraphy. Continental basins are influenced by multiple factors such as tectonics, climate, and sediment supply, leading to complex sedimentary patterns, rapid lateral and vertical changes, and frequent lake level fluctuations. These characteristics make the formation of continental sequence stratigraphy more intricate compared to marine settings. In China, researchers have developed specific models for continental basins based on boundary features, system domain limits, initial and maximum lake levels, and the presence of slope breaks. Key factors controlling terrestrial sequence development include lake level changes, tectonic activity, climate, and provenance supply. Among these, tectonics and climate are particularly significant as they directly influence lake levels. Common methods used in studying terrestrial sequence stratigraphy include outcrop analysis, experimental studies, well log interpretation, seismic analysis, and numerical simulations. Sequence stratigraphy is essential in all stages of hydrocarbon exploration, from initial targeting to field development. Effective porosity, which measures the proportion of connected pores in a rock, is crucial for evaluating reservoir quality. Unlike total porosity, which includes both connected and isolated pores, effective porosity reflects the actual storage capacity available for fluid flow. Most oil reservoirs have effective porosity ranging from 5% to 30%, with the most common range being 10% to 20%. Reservoirs with less than 5% porosity are typically considered non-economic unless fractures or other voids exist. In practice, porosity is often estimated using core samples, cuttings, and various geophysical techniques like electrical logging and nuclear magnetic resonance (NMR). NMR porosity is especially valuable because it directly measures pore fluids, offering advantages over traditional logging methods. However, in complex continental strata, discrepancies between NMR measurements and actual formation porosity can occur, affecting accuracy. To address this, researchers have developed tailored NMR logging methods suitable for China’s unique geological conditions. These approaches aim to improve the reliability of porosity data, ensuring better reservoir evaluation and exploration outcomes.
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