Well-log responses of overpressure and discussion about its mechanisms within Upper triassic-jurassic layer in yuanba area, sichuan basin, china

Abstract

Yuanba Area, located in Northeastern Sichuan Basin, has already been proved to be rich in natural gas in marine strata. Nowadays high yield of industrial gas has recently been discovered in the upper Triassic-Jurassic clastic rocks (T3∼J), which means a commercial breakthrough has been made in terrestrial formation. Interestingly, almost all the wells of high yield gas are in overpressure formation, and it seems that the higher formation pressure the more natural gas production is. To figure out the characteristic of overpressure formation's well-log responses and overpressure's mechanism, several experiments are carried out, including thin sections, scanning electron microscopy observation and physical simulation combined with forty pressure data and twenty-five wells log. The logging response characteristics of overpressure formation shows that mudstone density basically remains constant, resistivity increases obviously, however acoustic travel time slightly increases. Based on these characteristics, this study analyzes the reasons of this phenomenon in detail. Strong diagenesis and calcareous intercalation are supposed to be the main reasons for high resistivity, and at the same time, high acoustic time values are closely related to the increase of pore fluid pressure or the reduction of effective stress. Comprehensive analysis shows that the main mechanism in Yuanba Area is hydrocarbon generation and tectonic uplift. According to Biot's theory of poro-elasticity (Biot, 1941) and the state equation of ideal gas, this study estimates the contribution rate of each mechanism. The result shows that the contribution rate of hydrocarbon generation to the current overpressure is about 75∼85% with an average 82%; the contribution rate of tectonic stress extrusion is about 12∼24% with an average 18%. Even though the pressure amplitude caused by tectonic uplift is relatively small, the original overpressure distribution has been changed by the tectonic uplift to some extent, which formed the present pressure distribution.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015