Siderite diagenesis of the C1 Sands reservoir, Maui Field, Taranaki Basin, New Zealand

Abstract

The C1 Sands reservoir, located in the Taranaki Basin, New Zealand, contains the majority of the gas condensate reserves for the giant offshore Maui Gas Field. Siderite (FeCO3), which is present as a major cement throughout much of the C1 Sands, can be used as an indicator of depositional environment. Because siderite is one of the earliest minerals to precipitate in sediments, it is more likely to be influenced by the depositional environment than minerals that form later from more evolved pore waters. Siderite also provides information on early diagenesis and can be used as a tectonic indicator. The Maui core sediments show little correlation between the siderite and the environments of deposition between the different Maui cores, implying that there was limited lateral pore water fluid flow during the Mid to Late Eocene. Due to the widespread nature of the gas condensates now present in the C1 Sands reservoir, the fluid flow now is also widespread. Two different zones of siderite are present throughout the C1 Sands. One almost pure FeCO3 and the other containing much more impurities (Mg, Ca, Mn) are the result of precipitation in differing geochemical zones. The purer zone is the result of precipitation in the sulphate reduction zone and the impure zone from precipitation in the methanogenic zone.