Dolomitized Sandstones: A Marlin Field Gas-Reserves Assessment Problem

Abstract

Dolomite cement had been recognised within fluvio-deltaic facies sandstones of the Gippsland Basin Marlin field N-1 (Eocene) gas reservoir in the initial wildcat and two stepout wells. Although initially thought to be insignificant, the additional control provided by development drilling indicated that the degree and extent of dolomite cementation within the reservoir sandstones could be wide-ranging enough significantly to reduce reserves and influence reservoir performance. A study to define the area and degree of dolomite cementation within the reservoir showed that the distribution pattern within the 5 major sandstone units fell into 3 distinct groups. Whereas the 3 stratigraphically oldest (N-1.5, N-1.4 and N-1.3) units were only dolomitized within the northeastern portion of the field area, the N-1.2 unit was dolomite cemented field-wide. The youngest (N-1.1) unit, which unconformably overlies the truncated N-1.2 unit, is devoid of significant dolomite influence.Dolomitization of the N-1 sandstone units is interpreted to have occurred during a phase of estuarine-restricted marine shale deposition which followed the truncation of the N-1.2 sandstone unit. Although these estuarine dolomitic shales are absent by erosion at Marlin, they are present in the adjacent Tuna field area. Distribution of the dolomite cement within the reservoir sandstones was controlled by the subcrop pattern of the sandstone units during the deposition of the estuarine sediments.Petrological and x-ray diffraction information strongly suggest primary dolomite cementation, and not dolomitization of some pre-existing carbonate cement. The petrological, core analysis and geological information was integrated with the interpretation of wireline porosity logs to obtain reservoir unit average porosities and calculate reserves.The currently estimated initial N-1 reservoir dry gas reserves are 2.72 TSCF. If the reservoir sandstones were not dolomite cemented, reserves would be 3.15 TSCF, that is 14% greater.