Changes in reservoir heterogeneity and quality as a response to high partial pressures of CO2 in a gas reservoir, New Zealand

Abstract

The Kapuni Field is the largest onshore petroleum field in New Zealand and produces CO2-rich, gas (c. 40–45 mol% CO2). Diagenesis within the reservoir is dominated by CO2-related reactions that have resulted in localised precipitation of kaolin (abundant), quartz, calcite, dolomite, and siderite, along with localised generation of secondary porosity.Most of the CO2 in the reservoir is thought to have been sourced from intraformational coals, with subsequent up-dip migration to the crest of the Kapuni structure. During migration, CO2 will have dissolved into undersaturated pore fluids and the resulting acidic pore fluids catalysed feldspar (and minor carbonate) dissolution, thereby providing ions for precipitation of authigenic minerals. Timing of the diagenetic reactions, as determined by paragenetic observations and fluid inclusion analysis, suggests that both quartz and carbonate formed at a very late stage (>100 °C, corresponding to 0–4 Ma), which is consistent with modelled maturation and expulsion of CO2 from intraformational source rocks (5 Ma to present). The carbon isotope composition of carbonate cements (median δ13CPDB −12.4‰) is similar to that of the reservoired CO2 (δ13CPDB −14.5‰), and this is supportive of late-stage cement formation with carbon sourced primarily from thermal maturation reactions.The degree of CO2-related reactions observed within the Kapuni Field appears dependent upon grain size, lithofacies and stratigraphic position in the reservoir. Coarse-grained, high-energy facies have undergone the most feldspar dissolution, coupled with formation and local preservation of secondary porosity. This is interpreted to be due to preferential migration of large volumes of CO2-rich fluids through permeable beds, thereby further enhancing reservoir quality. By comparison, much less feldspar reaction but locally significant authigenic mineral precipitation occurs within finer-grained, lower-energy sandstone facies. This is considered to be due to relatively low permeability (prior to CO2 circulation), restricting the degree of fluid circulation, and resulting in net mineral precipitation. Authigenic kaolin has precipitated close to the mineral source, whilst ions required to precipitate calcite/dolomite cements are likely to have migrated further, resulting in a complex distribution of carbonate cements, with pervasive cements observed at gas/water contacts.Results from this study demonstrate how CO2-related reactions are dependent on grain size and mineralogy. We show that these reactions can affect vertical heterogeneity of a reservoir by increasing the degree of compartmentalisation through development of ‘sweet spots’ (due to mineral dissolution) and tight beds (due to mineral cementation).