Paleozoic Organic Rich Lacustrine Mudstones of New Brunswick and Nova Scotia, Depositional Models and Variability, Implications for Reservoir and Generated Fluid Properties

Abstract

Paleozoic organic rich lacustrine mudstones are important source rocks within the Maritimes Basin, but also form shale gas reservoirs and oil shale deposits. The 100–400m thick Lower Carboniferous Frederick Brook Member of the Albert Formation within the Moncton Basin of the Maritimes Basin of southeastern New Brunswick is comprised of a thick succession of mainly lacustrine clastics and carbonates. Organic rich mudstones occur at several stratigraphic levels, both intercalated with relative deep water lake deposits and also with relative shallow shoreline proximal deposits. Thus organic content of the deposits was likely main related to organic productivity and not to water depth, i.e. anoxia and detrital dilution. Organic matter content and composition changes quite significantly vertically, with RockEval analysis showing that surprisingly the shallower water compared with the deeper water organic rich mudstones generally have higher organic content (8–12% with a maximum of 29% versus 6–8%), slightly higher Hydrogen Index (850–950 versus 700–850) (Lynch, 1999). These difference in organic content and composition is reflected in drastic difference in oil shale yield productivity tests with the shallow water deposits in the uppermost part of the Frederick Brook Member having 2 to 3 times higher yields, average yield of 90–100 liters/ton of 30 API oil (Lynch 1999; Macauley 1984). Relative high lateral variability between wells in organic content, composition and thereby oil shale yields of the organic rich mudstones reflect the relative shallow water depositional setting and observed lateral changes in sedimentary facies. Similar high yield shallow water organic rich lacustrine mudstones are also present in oil shale deposits of the Pennsylvanian Pictou Group by Stellarton in Nova Scotia, where they are intercalated with grey coastal plain mudstones, coarse grained fluvial deposits and thick coal seams, in cases directed overlain by thick coals. The hydrogen rich oil shale deposits are rich in type 1 kerogen, dominated by Botryococcus-type telalginite algae remains (Macauley 1984). The findings that the shallow water organic rich mudstones have the highest hydrogen content not only have a large impact for evaluation of oil shale potential of the succession, but also for selection of units for the ongoing shale gas exploration of these deposits in the deeper part of the Moncton Basin, i.e. pore development during maturation and fluid content.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015