Peter Tingate

The origin of overpressure in the Carnarvon Basin, Western Australia: implications for pore pressure prediction

Porosity–effective stress analysis of 37 wells in the Carnarvon Basin, Australia, has identified 12 wells where the overpressure was generated by disequilibrium compaction and four wells where the overpressure was generated by fluid expansion. Disequilibrium compaction was identified as the dominant overpressure-generating mechanism in wells along the Rankin Trend as far south as Gorgon 1 and Spar 1. Fluid expansion was identified as the dominant mechanism of overpressure generation in wells along the Barrow Trend and around the Alpha Arch. Disequilibrium compaction-generated overpressures occur, as would be expected, where the Tertiary sediment thickness is greatest and fluid expansion overpressures where the Tertiary is thinnest. Indeed, where the N-1 (35 Ma) reflector is greater than c. 1500 m below seabed, disequilibrium compaction overpressures are observed and where itis shallower than c. 1500 m, fluid expansion overpressures are observed. Acoustic log-based pore pressure detection using Eatons (1975) method yielded accurate estimates with an exponent of three in Wilcox 1 where the overpressure was generated by disequilibrium compaction, and an exponent of six in Bambra 1 where the overpressure was generated by fluid expansion. If the Eaton (1975) technique is to be applied to seismic processing velocities for pore pressure prediction, then an exponent of three can be used where the N-1 horizon is greater than c. 1500 m and an exponent of six where the N-1 horizon is less than c. 1500 m.

Origin of quartz cement in the Tirrawarra Sandstone, southern Cooper Basin, South Australia

ABSTRACT Quartz cement is the most abundant authigenic mineral in the fluvio-deltaic Tirrawarra Sandstone and plays an important role in controlling reservoir quality. Quartz cement ranges from 0 to 19% and is controlled by the original sandstone composition. Petrographic, fluid inclusion, electron microprobe and cathodoluminescence (CL) data from the quartz cement indicate multiple stages of cementation at different temperatures and suggest more than one silica source. CL observations indicate up to six stages of quartz cement in some samples. The stages of quartz cement can be classified into three zones: an innermost zone of brown-luminescing cement (Z1), a middle zone of bright blue-luminescing cement (Z2) and an outer zone of brown-luminescing cement (Z3). Dead oil or bitumen is trapped b tween Z2 and Z3, indicating that Z3 formed after oil migration commenced. Measurements of homogenization temperatures from fluid inclusions in quartz overgrowths indicate that quartz cement precipitated over a temperature range of 65 to 130°C. Z1 quartz cement formed between 65 and 80°C and Z2 cement was precipitated between 80°C and 100°C. Z3 quartz cement formed later at temperatures around 130°C. Microprobe analysis shows a consistent variation in aluminum between each quartz cement zone. The average aluminum content for Z1, Z2, and Z3 is respectively 237, 538, and 58 ppm. Fluid-inclusion precipitation temperatures and aluminum content have been used to help identify the silica sources for different zones of cement. A likely source of silica for Z1 quartz cement is early alteration of feldspar grains. The bright blue CL color in the Z2 quartz cement is related to its high aluminum content. The source of silica for this cement is likely to have been late dissolution of feldspar grains by acidic fluids generated during kerogen maturation. Late quartz cement (Z3) has the lowest aluminum content, which is similar to that of detrital quartz grains in the Tirrawarra Sandstone. Consi ering the temperature of precipitation, very low aluminum content, and the presence of Z3 cement in facies prone to stylolitization, the silica source for the cement is likely to have been pressure solution of detrital quartz at stylolites and grain contacts.

CO2-related seal capacity enhancement in mudstones: evidence from the pine lodge natural CO2 accumulation, Otway Basin, Australia

Publisher Summary There is a clear correlation between the occurrences of late stage siderite cement near fracture edges and the enhancement of the seal capacity in the Belfast Mudstone. Isotopic evidence shows that the siderite has mineralogically trapped CO 2 through CO 2 -brine-rock interactions within the fractures and pore space of the rock. The precipitation of the CO 2 in the form of siderite has enhanced the sealing capacity of the rock. Understanding CO 2 -related reactions in low permeability mudstones is necessary to determine seal effectiveness in CO 2 geological storage. The Pine Lodge Field contains natural CO 2 within the Belfast Mudstone, a seal for many gas accumulations throughout the Otway Basin, Australia. The Pine Lodge CO 2 accumulation occurs in fractured shale and interbedded siltstone/sandstone. This natural analogue for CO 2 storage provides evidence of seal modification by CO 2 . Infiltration of CO 2 into the seal rock is strongly restricted by low permeability. In the case of the Belfast Mudstone the CO 2 -brine-rock interaction has resulted in the enhancement of seal capacity via mineral precipitation filling fractures and pore space. However, this has only occurred due to the appropriate chemical conditions during CO 2 interaction, including the availability of Fe 2+ cations for siderite formation. A negative aspect of CO 2 interaction is that the seal capacity could also decrease through labile mineral dissolution with the lack of appropriate cations.

Basin-scale fluid flow in the Gippsland Basin: implications for geological carbon storage

Petroleum systems analysis has been carried out to better understand the geological CO2 storage potential of the Gippsland Basin. From a regional perspective, the hydrocarbon migration architecture of the basin is interpreted to be dominated by two highly connected, filled-to-spill, hydrocarbon fairways; the northern (gas-dominated) and southern (oil-dominated) fill-spill chains, forming a convergent system that extends onshore along the Golden Beach Fill-Spill Chain (GBFSC). A separate oil-dominated Fill-Spill Chain, the Dolphin-Perch Fill-Spill Chain (DPFSC), is identified offshore to the southwest. Two broad flanking provinces, the Northerly Migration Province (NMP) and Southerly Migration Province (SMP), are also identified. Both provinces have broadly ramp-like geometries and relatively low dips. Migration across these provinces is not focused, and hence multiple pathways are present across a wide area. An understanding of the hydrocarbon systems in the basin can be used for characterising the potential for CO2 storage. Previous studies have shown that the top seal potential of the offshore Gippsland Basin is suited to geological carbon storage and that large areas are prospective as storage regions. However, the linked nature of the fluid flow systems and the focused fluid flow fairways between areas of high storage potential and leaky systems onshore will require both a good regional geological understanding and informed resource management.