Marine Source Rocks in Platform-Ramp Settings—An Example From the Early Permian Arckaringa Basin, South Australia

Abstract

Marine source rocks can accumulate in platform-ramp settings during second order stratigraphic cycles, when extensive shallow seaways flood continental regions. Many petroleum geologists will be familiar with the association of maximum organic carbon content in marine source rocks with condensed sections at maximum flooding surfaces. However in platform-ramp settings, maximum organic carbon content can occur just above the marine transgressive surface and decline gradually to the maximum flooding surface. Understanding these relationships is of particular relevance for those evaluating or working unconventional shale plays. The Early Permian Arckaringa Basin succession records an ~20 my second order stratigraphic cycle commencing with a basal glacigene unit, overlain by brackish to restricted marine mudstones, and topped by deltaic and fluvio-lacustrine-peat swamp sediments. The succession includes thick, organic rich mudstones with Type II kerogen, deposited during the marine transgression, with a prominent gamma-ray peak marking the maximum flooding surface. Published Permian oxygen isotope curves of both low and high palaeo-latitudes indicate overall warming of the global climate during the Early Permian. The maximum flooding surface in the Arckaringa Basin corresponds to peak ocean temperatures indicated by these curves. Organic carbon profiles in the thick Arckaringa Basin source rocks are typical of marine organic rich rocks that accumulate in platform-ramp settings. Marine source rock intersections in three Arckaringa Basin wells show a consistent pattern of maximum organic carbon content just above the transgressive surface, declining gradually to the maximum flooding surface. The decreasing preservation of organic matter is interpreted to result from increasingly vigorous ocean circulation as the sea-level rises.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015