Processes involving the formation of Organic Facies for Oil and Gas-Condensate Source rocks in the Late Triassic to Early Cretaceous Sediments from the Central Atlantic Conjugate Margin

Mukhopadhyay (Muki), Prasanta K.¹ (1) Global Geoenergy Research Limited, Halifax, NS

The organic facies of the Late Triassic to Early Cretaceous sediments from various exploratory and DSDP-ODP wells of the rifting and drifting phase of the Central Atlantic Conjugate Margin (Nova Scotia and Morocco) have been evaluated based on organic petrology and geochemical fingerprinting. The anoxicity of the Late Triassic and Early Jurassic synrift sediments is mainly controlled by salinity gradients and admixtures of terrestrial and lacustrine phytoclasts forming either oil prone Type I-II or gas-condensate prone Type II-III or III source rocks.

The characteristics of the organic sediments from Mid-Late Jurassic (Bathonian-Callovian to Kimmeridgian) to Early Cretaceous (Berriasian-Hauterivian) age is either related to the formation of mid-ocean anoxia created by bacterial degradation of algal organic matter or by the development of in situ anoxicity due to the mass imbalance of the abundance of organic matter input and consumption by bacterial degradation. The organic input is mainly created by a turbidity mass wasting in the Moroccan Margin whereas both shallow marine or deltaic and turbidity flows could be the possible factors in the Nova Scotia Margin. The formation of oil (Type II) or gas-condensate source rock in both organic facies is controlled by the settling velocity of the phytoclast precursors and formation of fecal remnants of zooplanktons and the abundance of oxygen-functional group with hydrocarbon bonds within the organic matter. The variability of salt emplacement from the diapiric to gliding salt canopy stages could be closely related to the source rock anoxicity during the Late Jurassic to Early Cretaceous time.