Apparent overconsolidation of mudstones in the Kumano Basin of southwest Japan: Implications for fluid pressure and fluid flow within a forearc setting

Abstract

The Kumano Basin is located in the Nankai Trough subduction zone of southwest Japan. During the past 1.6 million years, approximately 800 meters of sandy turbidites and hemipelagic mud were deposited near the distal edge of the forearc basin, at Site C0002 of the Integrated Ocean Drilling Program. Constant-rate-of-strain consolidation tests yield estimates of in situ permeability that range from 2.6 Â 10 À17 m 2 to 2.5 Â 10 À18 m 2 ; overconsolidation ratios range from 1.7 to 2.6, and values of the compression index range from 0.39 to 0.78. Several processes contributed to the apparent overconsolidation. Strata dip toward land, and pore fluids probably migrate up-dip and vent along a bathymetric notch near the seaward edge of the basin. Efficient lateral drainage through sandy turbidites has kept pore pressures within interbeds of mudstone at (or close to) hydrostatic. In addition, alteration of dispersed volcanic glass, precipitation of authigenic clay minerals, and collapse of random grain fabric has probably strengthened the bonding among grains. Cementation is particularly likely within the lower basin (unit III), where values of porosity remain anomalously high. If fluid overpressures (and underconsolidation) exist anywhere within the basin, the most likely loci are where sandy turbidites terminate against impermeable mudstones along landward-dipping on-lap surfaces. Those types of on-lap geometries, in addition to structural closures, might provide promising targets for oil/gas accumulation in other forearc basins, particularly where petroleum source rocks have been buried to the optimal depths of catagenesis.

Analysis of normal fault populations in the Kumano forearc basin, Nankai Trough, Japan: 2. Principal axes of stress and strain from inversion of fault orientations

“…In all cases the effective stress ratio (s 0 3 /s 0 1 ) is 0.27, satisfying the failure criterion for normal faulting given the internal and residual friction angles inferred from conjugate faults. Previous studies have shown that pore fluid pressures are near hydrostatic through much of the Kumano Basin [Guo et al, 2013;Saffer et al, 2010Saffer et al, , 2013. This allows us to quantify the magnitude of effective stresses, based on our inversion results and the total overburden stress (s v ) defined by integration of density logs.…”

In situ stress and pore pressure in the Kumano Forearc Basin, offshore SW Honshu from downhole measurements during riser drilling

“…For this range of C v , we predict maximum pore pressures of 0.05-1.6 MPa at the base of the basin section, corresponding to pore pressure ratios of l* = 0.004-0.13, in good agreement with the observations (where l* = (P f À P h )/(P l À P h ), and P f , P h , and P l are the pore fluid pressure, hydrostatic pressure, and lithostatic pressure, respectively) ( Figure 10). Because this simplified model assumes only one-dimensional consolidation and fluid flow, any lateral drainage along dipping strata would further enhance drainage [e.g., Guo et al, 2013]. If the higher values of permeability obtained from the SP tests are representative of the formation, the corresponding values of C v would bẽ 2.5 Â 10 À7 to~6 Â 10 À6 m 2 /s and would yield even lower pressures (cf.…”