Predicting marine organic-rich deposits using forward stratigraphic modelling: The Jurassic Najmah source rock – Case study

Abstract

Predicting the distribution and heterogeneity of marine Mesozoic organic-rich rocks is a challenging task that requires multi-disciplinary data integration supported by innovative numerical modelling. This study aims at investigating the factors controlling marine organic matter production, accumulation, and preservation along the eastern Arabian intrashelf basins. Such types of basins exhibit an enhanced lateral and vertical facies variability caused by the interplay between shallow and deeper water sedimentation. The work focuses on characterizing the Mid-Upper Jurassic Najmah Formation that presents source rock, reservoir, as well as seal characteristics along Kuwait. It comprises high maturity levels (>0.6% Ro) and TOC values ranging between 2 and 30%. Forward stratigraphic modelling that includes organic matter production, transport, and preservation (i.e., degradation and dilution) was utilized. Models corresponding to two end-member basin types (restricted and fully open marine) have been simulated to mimic the organic matter deposition within a 3rd-order sequence stratigraphic framework. Simulation outputs provide a qualitative and quantitative assessment of organic matter characteristics (TOC, HI, and OI) at the time of deposition. Results show that (1) a combined variable eustatic sea level change and tectonic subsidence are the main driving mechanisms for sediment accommodation during the Mid-Upper Jurassic time, (2) strong anoxic conditions in the restricted basin simulations promote the preservation of organic-rich deposits in the distal northern sectors while (3) the influence of open marine bottom current oxygenation is not expected to impact the relatively shallower conditions affecting the intrashelf basin (<200 m depth), (4) organic matter dilution can be observed along the southernmost sectors of Kuwait due to high sedimentation rates related to the development of gravity driven deposits. Sensitivity analysis on input parameters (i.e., primary productivity vs water depth) supports the partially restricted basin conditions with occasional connection to the open ocean during eustatic sea level rise. This study contributes to the improvement of basin models in conventional as well as unconventional petroleum system exploration.