2013 Volume 122 Issue 1 Pages 90-115
The sonic log provides a good compaction curve that shows the porosity of sediments reducing with depth. Sonic-determined compaction curves from 150 wells in the Niigata Sedimentary Basin are classified into three types: Nagaoka-heiya, Niigata-heiya, and Takada-heiya. The Nagaoka-heiya type is the most common compaction curve in Niigata with Δt (formation interval transit time) decreasing continuously with depth. The Niigata-heiya type shows less compaction with Δt decreasing more gradually. The Takada-heiya type shows more compaction than the Nagaoka-heiya type at depths greater than 1500 m. Compaction is controlled mainly by burial depth, but also by time and temperature. No clear relationships are observed between compaction rate and overpressure or sand/shale ratio.
The Niigata-heiya-type wells, which show lower compaction, are distributed in areas with young and rapid sedimentation of more than 2000 m/m.y. Rapid sedimentation may cause insufficient dewatering from pore spaces, in turn causing overpressure and undercompaction. However, the Niigata-heiya-type wells do not show distinctive overpressure, so insufficient dewatering does not explain low compaction. Mechanical rearrangement of sedimentary grain framework takes a relatively long time, and 1 m.y. is not long enough for complete rearrangement of 2000 m thick sediments.
Higher temperature accelerates chemical processes such as diagenesis and cementation. Wells of the Takada-heiya type are believed to have been exposed to higher thermal gradients between 16 Ma to 9 Ma, causing rapid compaction and higher maturity of source rocks. Several normal compaction trends have been established for each area in the Niigata Basin.
The amount of eroded strata at each well was estimated by referring to the normal compaction trend, and an erosion map of the Niigata basin was made. The hypocenter of the 2004 Mid-Niigata Prefecture Earthquake was located 13 km below the Araya Anticline. Estimated erosion is concordant with the shape of the anticline and deformation of the earthquake. Its cross-section and geohistory diagram indicate the Araya Anticline was initiated at about 1.3 Ma by an inverse movement of the Inokurayama Fault, where a normal fault was reactivated as a reverse fault, and 1600 m of sediments were subsequently eroded.