Porosity refers to the ratio of the pore volume within a rock (or the space not occupied by solid material) to the total volume of the rock. It is a critical parameter in understanding the storage capacity of fluids such as oil and gas. The study of porosity differs significantly between continental sequence stratigraphy and passive continental margin marine sequence stratigraphy. In continental basins, sedimentation is influenced by multiple factors, including tectonic activity, climate, and lake level fluctuations. These factors lead to complex sedimentary patterns, rapid lateral and vertical changes, and frequent lake-level variations, making the formation of sequence stratigraphic models more challenging. Chinese researchers have developed specific sequence stratigraphic frameworks tailored to the unique conditions of Chinese basins, taking into account boundary characteristics, system domain features, and lake surface dynamics. Key controlling factors for terrestrial sequence development include lake level changes, tectonics, climate, and sediment supply, with tectonics and climate being particularly significant. Research methods include outcrop analysis, experimental studies, well log interpretation, seismic analysis, and numerical modeling. Sequence stratigraphy plays a vital role in all stages of oil and gas exploration, from initial assessment to field development.
In oil and gas exploration, effective porosity is defined as the ratio of connected pore space to the total rock volume. This is different from total porosity, which includes both connected and isolated pores. Typically, effective porosity is 5–10% lower than total porosity. Most oil reservoirs have porosity ranging from 5% to 30%, with the most common range being 10% to 20%. Reservoirs with porosity below 5% are generally considered non-economic unless natural fractures or vugs exist. Based on field experience, reservoir rocks can be classified according to their porosity levels:
- 0–5%: Non-value porosity
- 5–10%: Poor porosity
- 10–15%: Medium porosity
- 15–20%: Good porosity
- 20–25%: Excellent porosity for reservoir evaluation
Porosity is a key parameter in reservoir evaluation. Nuclear magnetic resonance (NMR) logging provides an accurate measure of porosity by detecting fluid-filled pores, offering advantages over other well logging techniques. However, in China’s complex continental strata, NMR porosity readings often differ from actual values, sometimes significantly. This discrepancy affects the reliability of NMR logging. To address this, extensive laboratory experiments using artificial and natural rock samples have led to improved porosity logging methods suitable for China’s geological conditions. Given the complexity of these formations, it is recommended that regional-specific analyses be conducted, and appropriate NMR logging techniques be selected to ensure more accurate porosity measurements.
Porosity is commonly measured in the lab using core samples or cuttings. Additionally, several qualitative methods are used to estimate porosity, such as electrical resistivity measurements, radioactive logging, and others. These techniques help geologists better understand the subsurface and optimize hydrocarbon recovery strategies.
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