Pore and mineral controls on petrophysical anomalies in undercompacted mudstones

Abstract

Abstract In mudstones, undercompaction inhibits grain rearrangement, preserving pores and generating anomalous petrophysical responses. These anomalies typically manifest in geophysical exploration as abnormal acoustic and resistivity logs. However, the microstructural differences between undercompacted and normally compacted conditions remain unclear, and the mechanisms linking pore preservation to petrophysical property anomalies are not well understood. Neogene mudstones from the Bohai Bay Basin serve as a case study, integrating well log analysis, experimental research, and digital rock physics methods to investigate pore structure and petrophysical property responses in mudstones under different compaction conditions. Although well log analysis exhibits non-uniqueness, rock physics experiments conclusively demonstrate that these anomalies are related to mudstone undercompaction. Results reveal that undercompacted mudstones exhibit significantly higher porosity (average 20.54%) than normally compacted mudstones (average 14.26%), along with a higher clay content and a well-connected network of interaggregate pores. The primary cause of their low resistivity is the combined effect of this interconnected macro-pore system and high clay content. Density-acoustic transit time crossplots confirm that overpressure in the study area originates from undercompaction rather than fluid expansion or diagenesis. Digital rock simulations quantitatively reveal that porosity has approximately twice the effect of clay content on resistivity reduction, and further demonstrate that undercompacted mudstones generate low acoustic impedance anomalies similar to sandstone reservoirs but are distinguished by significantly higher Vp/Vs ratios, providing a robust seismic discriminant. These findings clarify the pore and mineral controlled origins of petrophysical anomalies in undercompacted mudstones and offer practical criteria for their accurate identification in geophysical exploration, thereby supporting improved reservoir interpretation and drilling hazard assessment.