The evolution of halite, solid bitumen and carbonate in the petroleum system of the South Oman Salt Basin

摘要

This thesis deals with diagenetic and structural processes in the Late Precambrian to Early Cambrian Ara Group petroleum system of the South Oman Salt Basin (SOSB) and the Ghaba Salt Basin (GSB). The Ara hydrocarbon play is constituted by partly highly overpressured up to 150 m thick carbonate bodies (so-called ‘stringers’), which are source and reservoir rock at the same time. The stringers are fully enclosed by thick Ara salt diapirs, which form the bottom, side and top seal of this unconventional hydrocarbon system at burial depths of ~ 3 to 6 km. The evolution of reservoir quality of the stringers in the SOSB was investigated with special emphasis on the cementation of porosity by halite and solid bitumen as well as on the sealing capacity and rheology of the surrounding Ara salt. In addition, these findings were compared with data from the Ara Group, constituting six surface-piercing salt domes in the GSB (Northern Oman). Based on detailed microscopic investigations using different techniques, a number of geochemical analyses were carried out, involving organic geochemistry and correlated solid bitumen reflectance, stable isotopes of carbonates and bromine geochemistry of halite. Halite cementation is one of the greatest exploration risks in the Ara stringer play. An early phase of halite cementation pre-dates solid bitumen in pores of facies with initially favourable reservoir properties. In most cases, this early halite plugs all available porosity in the uppermost parts of the stringers. Geochemistry of the early halite revealed significantly high contents (up to 280 ppm) of bromine. The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation due to seepage reflux of highly saturated brines during deposition of the following rock salt interval. Petrographically late halite post-dates solid bitumen in pores and fractures and is depleted in bromine (down to 170 ppm) as a result of solution-reprecipitation of early halite. Solid bitumen represents another reservoir-deteriorating phase in the Ara stringers and occurs as pore- and fracture-lining cement. It mainly formed by thermal cracking due to the influx of external hydrothermal fluids as deduced from a very heterogeneous distribution of maximum paleo-temperatures throughout the Ara stringer intervals. This is supported by the presence of coke-like solid bitumen, which indicates paleo-temperatures up to 380°C and by hydrothermal minerals in veins. The influx of these fluids into the carbonate stringers is considered to represent a major contribution to their strong overpressures. These high fluid overpressures were partly released into the over- and underlying Ara salt intervals as evidenced by black hydrocarbon-stained salt cores, which indicate repeated loss of its sealing capacity. Microstructures of this black salt show solid bitumen-impregnated grain boundaries and microcracks and also evidence for crystal plastic deformation and dynamic recrystallization. Subgrain size piezometry indicates a maximum differential paleo-stress of less than 2 MPa. Under such low shear stress, laboratory-calibrated dilatancy criteria indicate that oil can only enter the rock salt at near-zero effective stresses, where fluid pressures are very close to lithostatic. This means that the oil pressure in the carbonate reservoirs increased until it was equal to the fluid pressure in the low but interconnected porosity of the Ara Salt plus the capillary entry pressure. These conditions define the limits to the sealing capacity of halite, above which the permeability can increase by many orders of magnitude. Sealing capacity is regained, if the fluid pressure drops below sigma 3, at which point rock salt will re-seal to maintain the fluid pressure close to lithostatic values. Triaxial deformation experiments of the Ara salt revealed very similar creep behaviour compared to the most common deformed halite rocks with steady state creep at strain rates of 10-7 1/s, which is orders of magnitude higher than expected in-situ rates, except perhaps during borehole closure-related creep during drilling. Microstructural investigations of Ara salt from the subsurface (SOSB) and the surface (GSB) indicate growth of chevron and hopper crystals in marginally shallow marine brine pools. During subsequent passive diapirism, the salt started to dynamically recrystallize, accompanied by fluid-assisted grain boundary migration. Subgrain size piezometry of the Ara salt revealed that highest differential paleo-stresses (up to 5 MPa) occur in the narrow stem of a diapir. It is in this structural level, where the intra-salt stringers underwent most intense deformation as observed by tectonic breccias and veins of the surface-piercing carbonates in the GSB. Stable isotopes of these veins indicate a meteoric signature, suggesting deformation under diagenetic open conditions due to salt dissolution prior to final piercement of the surface.