ABSTRACT: Origin of Hydrocarbon-Associated Dolomites, Early to Middle Cambrian of Georgina Basin, Northern Territory, Australia

NICOLAIDES, STELIOS, and JOHN K. WARREN,* National Centre for Petroleum Geology and Geophysics, Adelaide, Australia

Oil and gas holes drilled by Pacific Oil and Gas Pty. Ltd. have intersected live oil shows of various degrees of quality. In core, these hydrocarbon indications range from individual oil-filled vugs to meter-thick zones of oil-saturated microcrystalline dolomite. The "best" oil shows usually occur in the tighter dolomitized lithologies in intercrystalline, vuggy, or occasional fracture porosity. Of the wells investigated in this study, oil shows were most common in Phillip #2, followed by MacIntyre #1, and Baldwin #1. Live oil in these wells was observed in several different lithologies with shows typically occurring in the basal Cambrian carbonates. The unbiodegraded oils range in composition from paraffinic to aromatic intermediate, with aromatic compositions that reflect their deri ation from marine carbonate source beds. No oil was recovered from drill stem tests conducted on these wells. Hence the relationship that controls hydrocarbon occurrence in these wells is thought to be intimately related to the diagenetic history of these potential reservoir carbonates. This study was conducted to detail this diagenetic history.

The dolomite consists of shallow water peritidal and deeper water subtidal (reefal and lagoonal) lithologies. Three dolomite types are recognized: synsedimentary, replacement, and cement. Synsedimentary dolomite was derived from syndepositional brines in a sabkha environment. Finely crystalline anhydrite was deposited with this dolomite. Replacement dolomite is volumetrically the most widespread. It varies in crystal size from fine to coarse and forms xenotopic to hypidiotopic crystal mosaics. Under cathodoluminescence, it is unzoned and has a mottled appearance. Dolomite cement is coarsely crystalline and forms idiotopic to hypidiotopic crystal mosaics. Some crystals show undulose extinction and have curved planes. Under cathodoluminescence, the dolomite shows a succession of non-, b ightly-, and dully-luminescent zones. Coarsely crystalline anhydrite postdates this cement type.

Geochemically, the synsedimentary dolomites (delta{18}O[(PDB)] values between -8.61% to -5.35%) and the dolomite cements (delta{18}O[(PDB)] values ranging from -14.34% to -10.80%) are distinctive. Replacement dolomites have a wide range of d{18}O values from -10.90 to -3.30% (PDB). The delta{13}C values of all three dolomite types are quite similar.

The end result of diagenesis is a pervasively dolomitized carbonate altered by evaporite-related, dolomite-saturated diagenetic fluids. Porosity in the sequence is low and composed of secondary intercrystalline and vuggy pores. To obtain subsurface porosity of reservoir grade and extent will require identification of areas with either partial preservation of intercrystalline/mouldic porosity on the marine side of the evaporite beds or the preservation of burial-stage porosity along conduits that were permeable at the time of oil emplacement, such as fault-associated conduits or along sharp lithological boundaries (both depositional and diagenetic).

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)