Petroleum source rock potential of North Cape Formation (Late Cretaceous) coaly sediments, Taranaki Basin

摘要

The Wainui and Puponga coal measure members of North Cape Formation (Late Cretaceous) are potentially important source rock units within the Western Stable Platform and Pakawau, Maui and Manaia sub-basins of Taranaki Basin. This paper presents a regional study of the petroleum potential of their coaly sediments, based mainly on seven widely spread wells, and emphasises the identification of depositional controls on genetic potential and oil-proneness. Most of the coals are mature enough to have commenced generation of oil, but only in Taranga-1 have they begun expulsion. The total genetic potential and oil-proneness of the coaly sediments are generally underestimated by the Rock-Eval parameters S1+S2 and HI, but projection of these parameters to the inferred onset of oil expulsion (Rank(S) 12.5) allows the true potential and oil-proneness to be estimated. All coaly sediments with TOCs above ～2.5% have good genetic potential. The three primary lithologies - carbonaceous (coaly) mudstone, shaly coal and coal - form a continuum of Type III source rocks, all parts of which contribute to the total genetic potential of the coal measures in proportion to their mean organic content and total volume. The continuum is estimated to range in HI at the onset of expulsion, from 235-420 mg HC/g C_(org), indicating mixed gas- and oil-prone to oil-prone kerogen. Variable marine influence throughout coal measure deposition has resulted in pronounced lateral and vertical variations in the genetic potential and oil-proneness of the source rock continuum. Wainui Member coaly sediments in Wainui-1 generally have greater genetic potential and are more oil-prone than lithologies of equivalent TOC in, for example, North Tasman-1, Tane-1 and Taranga-1, which are generally less marine-influenced, as judged by coal sulphur content and dinoflagellate abundance. Similarly, wide variations in volatile matter, atomic H/C and HI - and therefore in oil-proneness - within the North Tasman-1 and Tane-1 sequences in particular, are attributed to variations in the degree of marine influence at these sites.