Fluvial/Lacustrine Diagenesis: Significance for Hydrocarbon Production and Entrapment in the Carboniferous Albert Formation, Moncton Basin

J. P. A. Noble, A. H. Chowdhury, and H. Yu

The Carboniferous Horton Group Albert Formation sediments include lacustrine source-rock oil shales and fluvial porous reservoir sandstones. The petrography, stable isotopes, fluid inclusions, cathodoluminescence and minor/trace element chemistry are used to establish the diagenetic history and controlling factors. Early diagenetic calcite, quartz and albite cements with minor chlorite and kaolinite are variably present and related to depositional mineralogy and lake levels which controlled the porewater chemistry. Antitaxal veins occurring preferentially in shales are shown, from heavy ^dgrC¹³ values and fluid inclusions, to be related to methanogenesis, and may have formed is overpressured zones at depths up to 1 km. Later burial calcite and albite cements an extensive albitisation are related to mineral reactions which occurred during the phase of rapid subsidence at temperatures of 80° to 150° in the deepest segment of the basin, together with significant dissolution of carbonates and feldspars related mainly to organic acids generated by organic maturation processes. Mass balance calculations indicate that not enough organic matter was present to account for all the estimated secondary porosity and some evidence suggests that reactions between kaolinite and calcite\ankerite to produce chlorite, and mixed layer illite-smectite ordering reactions, may have produced significant secondary porosity. Burial history reconstructions and thermal modelling of the Albert FM. sediments using Arrhenius type maturity models and reflectance an rock-eval data suggest locally variable maturation and reservoir production related to the locally different fault tectonic histories characteristics of strike-slip lacustrine segmented basins. The Horton depositional cycle was followed by a major dextral transpression event with local faulting and inversion and deposition of a sequence of vein cements. the calcites, dolomites, ankerites and barites occurring in these veins appear to have formed during and after local tectonic uplift and during cooling from temperatures of about 150° to 80° in segments of the basin where local pre-Hillsborough uplift was greatest, producing 2 to 2.5 kms. of erosion, and mainly from circulating brines with CaCl-NaCl compositions and variable salinities of 8 to 27 wt. % as measured from fluid in lusions.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California