Hydrocarbon potential and depositional paleoenvironment of a Middle Jurassic succession in the Falak-21 well, Shushan Basin, Egypt: Integrated palynological, geochemical and organic petrographic approach

Abstract

The topmost Lower and Middle Jurassic formations of organic-rich clastics with minor carbonate deposits were recognized as very significant source/reservoir rocks in the North Western Desert of Egypt. Although several studies are devoted to their source rock characteristics and petroleum potential, the depositional paleoenvironment and paleoredox conditions are still poorly understood. In the current study, these sediments were assessed in terms of their organic matter richness, hydrocarbon generative potential and thermal maturity, total sulfur (TS) contents and organic petrographic characteristics. Depositional paleoenvironment was investigated using dinoflagellate cysts (dinocysts) and the composition of particulate organic matter along with organic petrographic characteristics. Redox conditions were assessed based on TS- total organic carbon (TOC) relationship and type of kerogen that was derived mainly from palynofacies analysis.TOC contents show fair to excellent generative potential of hydrocarbons mainly of kerogen type III (gas-prone). The thermal maturity, based on Tmax and Rr, indicates mature stage of oil window.Palynological analysis yielded poor to rich intervals of moderately-preserved, biostratigraphically useful dinocysts in the upper part of the Khatatba Formation. An assemblage of 81 species belonging to 54 genera of moderately diverse dinocysts, spores and pollen grains was identified. Freshwater algae, acritarchs and microforaminiferal test linings are sporadically documented within the studied interval. Based on the first downhole appearance (FDA) of the recovered marker dinocyts, the upper part of the Khatatba Formation was palynologically dated as late Bathonian-Callovian.The depositional environment was interpreted and three environments were deduced. The lower interval representing the topmost Ras Qattara and Yakout Red Shale Member was deposited in a fluvial-lacustrine environment, where oxic conditions prevailed as deduced from the color of reddish-brown sandstone and red shale. The middle interval spans Unit IIB, Kabrit and Unit III, which accumulated in a deltaic to shallow marine environment under dysoxic-suboxic settings. The upper interval covers Unit I and Unit IIA that have been deposited in a fluvio-deltaic to shallow marine with high paleoproductivity under suboxic conditions. One sample in the upper interval in Unit IIA reflected anoxic setting.