Understanding Of Overpressure Mechanisms For Gas Exploration In Tsushima Basin, Japan

Abstract

Geoscientists and drilling engineers face a common challenge in pore pressure predictions in frontier exploration projects as there are significant uncertainties about subsurface conditions without enough well controls. An exploratory well, Well A, was recently drilled in the offshore Tsushima Basin and encountered multiple gas-bearing zones including highly overpressured zones. The Tsushima back-arc basin consists of thick Miocene siliciclastic sediments and remains unexplored. Well A is a very important reference well to understand pore pressure and fracture gradient (PPFG) regimes necessary to proceed with subsequent exploration activities in this region. In this paper, we present a PPFG profile of Well A along with geological interpretations of complex overpressure mechanisms based on newly acquired well data (LWD, wireline and drilling data) and 3D seismic data. We also discuss an application of PPFG data for subsurface geological evaluations and future well designing. Well A penetrated a thick sequence of the Miocene K Group including several gas-bearing zones and encountered an unexpected gas cut with abnormally high-pressure at a little over 3,100mSS, where considerable time was spent for well control operations and consequently the well was plugged and abandoned. PPFG profiles of Well A were predicted by using seismic velocities in the pre-drill study, and carefully updated on a real-time basis by using LWD data at wellsite while drilling operations. In the post-drill stage, an integrated in-house PPFG study was conducted to understand the geological context of overpressures. As the result of the post-drill study, complex overpressure mechanisms are interpreted to be composed of 1) disequilibrium compaction, 2) chemical compaction and, 3) lateral transfer. Disequilibrium compaction is a primary mechanism starting around 1,400mSS as clearly indicated by the lithostatic-parallel pressure trend and density-velocity crossplot. Chemical compaction is implied as a secondary mechanism below 2,600mSS, where escalations in pressure gradient, temperature and illite-smectite ratio are observed. Lateral transfer is expected as a third mechanism around the TD (approx. 3,100mSS), where a tilted sandstone body is presumably distributed within overpressured shales. Although no lithological information (e.g. cuttings and LWD) is available around the TD, the existence of high-pressure gas sandstone body is suggested by such circumstantial evidence as: a) a distinct pore pressure gap between the TD gas and overlying shale zones as mud weight needed to increase from 13.0 to 16.0 ppg during well control operations, b) a deepwater channel-like feature and an updip termination of seismic event (i.e. indicative of pinch-out) interpreted in 3D seismic data, and c) no pit gain during gas cut suggesting only gas influx into borehole (i.e. no formation water, no caving). These comprehensive interpretations of overpressure mechanisms covering both shale and sandstone pressures are obviously useful for subsurface evaluations including seal capacity and potential gas column estimations as well as future well designing. To summarize, overpressure observed at Well A is primarily caused by disequilibrium compaction, and possibly influenced by the chemical compaction in the deep and high-temperature section of Well A. The abnormally high-pressure at the TD (approx. 3,100mSS) is probably associated with the lateral transfer mechanism caused by tilted gas sandstone reservoirs. The pre-drill PPFG prediction using seismic velocities was quite reasonable for shale (baseline) pressure, though the abnormal pressure at the TD was unpredictable. The optimum approach for sandstone pressure estimations along with conventional shale pressure predictions will be critical for the subsurface evaluation and well designing in the future.

AAPG Datapages/Search and Discovery Article #90324 © 2018 AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment, Perth, Australia, June 6-7, 2018