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. This concept is crucial in geology, especially in understanding how fluids like oil and gas are stored and flow through rock formations.
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 such as tectonic activity, climate, and lake level fluctuations. These basins often have complex sedimentary structures, rapid lateral changes, and significant vertical variations. Frequent lake advances and retreats lead to rapid changes in sedimentation patterns, making the formation of sequence stratigraphy more complicated and challenging to interpret. Chinese researchers have developed a unique sequence stratigraphic framework tailored to the specific conditions of Chinese basins, taking into account boundary characteristics, system domain boundaries, initial and maximum lake levels, and the presence of slope breaks. This model helps better understand the development of terrestrial strata.
The main factors controlling the development of terrestrial stratigraphic sequences include lake level changes, tectonics, climate, and provenance supply. Tectonic and climatic factors play a critical role in influencing lake levels directly. The methods used to study these sequences include outcrop analysis, experimental observations, well log interpretation, seismic data analysis, and numerical simulations. Sequence stratigraphy plays a vital role in all stages of oil and gas exploration—from initial discovery to field development.
In oil and gas exploration, effective porosity is a key parameter that determines the potential of a reservoir. It represents the proportion of connected pore space relative to the total rock volume. Effective porosity is typically 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 fractures or vugs are present. Based on field experience, reservoir rocks are 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 for reservoir evaluation
Nuclear magnetic resonance (NMR) porosity measurement is particularly useful because it responds only to fluid-filled pores, offering advantages over traditional well logging techniques. However, in China’s complex continental strata, discrepancies between NMR porosity and actual formation porosity are common, sometimes leading to inaccurate interpretations. Researchers have conducted extensive studies using artificial and natural rock samples to refine NMR logging methods suitable for these conditions. To improve accuracy, it is recommended that different regions be analyzed based on local geological conditions, and appropriate NMR logging techniques should be selected accordingly.
Porosity is usually measured in the lab using core samples or cuttings. Additionally, several qualitative methods such as electrical resistivity measurements and radioactive logging are also used to estimate porosity.
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