Origin of over-pressure in clastic rocks in Yuanba area, northeast Sichuan Basin, China

• Over-pressured mudstones in clastic formations are normally compacted.
• Strong gas generation accounts for the high magnitude over-pressures in mudstone.
• Oil cracking and pressure transference are the main causes for over-pressures in sandstone.
• Lateral tectonic compression and poroelastic effects also contribute to over-pressure generation.

Abnormally high pressures are widely developed in clastic formations with burial depths greater than 3500 m in Yuanba area, where the top depth of over-pressure zone encompasses different formations in different locations. With increasing depths, well logs response in mudstone to over-pressure zone show that no significant anomalies occur in sonic and density logs, suggesting that the over-pressured mudstones are normally compacted. Multiple evidences from well-log responses to over-pressure, low porosity, normal geothermal gradient, low sedimentary rate and velocity-density crossplots show that over-pressures in mudrocks were created by gas generation and not related to the disequilibrium compaction. Based on diagenetic evolution analysis, stratigraphic framework and gas accumulation model, pressure transference and oil-to-gas cracking are expected to be the main causes of over-pressure in sandstone. Because of the favorable sealing condition and gas-bearing properties, lateral tectonic compression has also been an important factor for over-pressure development and preservation since early Cretaceous.