Gamma ray spectrometry of a coarsening upward succession in outcrop - Albert Formation (Moncton Basin), Sussex N.B.

Abstract

The Albert Formation is the subject of ongoing hydrocarbon exploration in southeastern New Brunswick. Accordingly, the main purpose of this study was to use gamma ray spectrometry data obtained from a logged section of outcrop in order to aid geologic interpretation of the Albert Formation and correlation of surface and borehole (McCully#2) successions. The logged outcrop is located in southern New Brunswick, west of the town of Sussex and close to the 186 km marker on the northbound highway of Route #1. Based on detailed sedimentological analysis and gamma ray log trends, three packages are identified (A, Band C). These packages all contributed to an overall coarsening upward succession (0.0 - 139.5 ft.). Package A (0.0 - 13.5 ft.) comprises very fine-grained sandstone interbedded with siltstone and shale. This package is characterized by horizontal lamination, wavy parallel laminae and current ripples that are sporadically present. Package B (13.5 - 69.0 ft.) comprises mainly siltstone interbedded with laminated shales. The shales (18.0- 21.0 ft.) are characteristic of wavy parallel and low-angle parallel laminae. Also, there are some quiescent sedimentary structures such as compositional layering at 27.0 ft and the occurrence of soft sediment deformation. Package C (69.0-139.5 ft.) is predominantly sandstone that is variably interbedded with siltstone, shale and minor mudstone. Common sedimentary structures include ripple-cross laminae (88.5 ft), oscillation ripples (102.0 ft.), roots and horizontal lamination (120.0 ft.). Analysis of packages A, B and C indicates that these packages correspond to deposits formed in a lacustrine, pro-delta, delta front, and delta plain succession of environments. This suggests that the overall coarsing upward succession is indicative of a prograding delta front which results in coarser grained sediments (sandstone) being deposited over the underlying finer grained sediments (siltstone, shale and mudstone). The overall higher total gamma radiation occurs in package B and intermittently in package C, corresponding to mostly a lithology of either shale or siltstone. The overall lower total gamma radioactivity was mostly indicative of the sandstone units. This occurs mainly in packages A and C. The total gamma ray data from the Albert formation in McCully#2 well was compared to the gamma ray from the outcrop. The overall gamma ray signature of the logged outcrop is very similar to the total gamma ray signature between ~2090-2130 meters in the gas well which includes the 'C sand' gas reservoir. Since both gamma ray signatures in outcrop and borehole were similar, they are considered correlatable. This then suggests that the depositional environment of the 'C sands' in the sub-surface is also indicative of a prograding delta front.