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. Understanding porosity is crucial in geology and especially in oil and gas exploration, as it directly affects the storage capacity of hydrocarbons. The study of porosity varies significantly between continental sequence stratigraphy and passive continental margin marine sequence stratigraphy. Continental basins are influenced by multiple factors such as tectonic activity, climate, and sediment supply, leading to complex and diverse sedimentary patterns. These basins often exhibit rapid lateral changes, poor continuity, and significant vertical variations in sediment thickness. Frequent lake level fluctuations further complicate the formation of sedimentary layers, making the study of continental sequence stratigraphy more challenging. In China, researchers have developed a unique sequence stratigraphic framework tailored to the specific conditions of local basins. This model takes into account boundary characteristics, system domain boundaries, initial and maximum lake levels, and the presence of slope breaks. Key controlling factors for terrestrial sequence development include lake level changes, tectonics, climate, and sediment supply—especially tectonics and climate, which directly influence lake levels. Various methods are used in the study of terrestrial sequence stratigraphy, including outcrop analysis, experimental observation, well log interpretation, seismic analysis, and numerical simulations. Sequence stratigraphy plays an essential role in all stages of oil and gas exploration, from initial target identification to field development. In oil and gas exploration, effective porosity is the most relevant measure. It represents the proportion of connected pore space within the rock, as opposed to total porosity, which includes both connected and isolated pores. Effective porosity is typically 5–10% lower than total porosity, and most oil reservoirs have effective porosity ranging from 10% to 20%. Porosity below 5% is generally considered non-economic unless natural fractures or vugs exist. Based on field experience, reservoir rocks can be categorized according to their porosity levels: - 0–5%: Non-economic - 5–10%: Poor - 10–15%: Moderate - 15–20%: Good - 20–25%: Excellent Porosity is a key parameter in reservoir evaluation. Nuclear magnetic resonance (NMR) logging is particularly useful because it measures only fluid-filled pores, offering advantages over other logging techniques. However, in complex continental strata, discrepancies between NMR porosity and actual formation porosity can occur, affecting the accuracy of measurements. To address this issue, extensive research has been conducted using both artificial and natural rock samples. New porosity logging methods have been proposed to better suit China's continental formations, improving the reliability of NMR data. Due to the complexity of these strata, it is recommended that different regions adopt region-specific approaches and select appropriate NMR logging methods to achieve more accurate results. Porosity is commonly measured in the lab using core or cuttings samples. Additionally, several qualitative methods, such as electrical resistivity measurements and radioactive logging, are also used to estimate porosity.
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