Organic geochemistry of possible Middle Miocene–Pliocene source rocks in the west and northwest Nile Delta, Egypt

Abstract

An important step for any frontier basin source rock survey is to understand the natural variability of geochemical characteristics within the source intervals. No previous studies addressed the molecular characteristics of organic matter or the source rock potential in the west and northwest onshore/offshore portion of the Nile Delta province. For this reason, the current work used geochemical proxies, including Rock-Eval/TOC screening analyses, gas chromatography–mass spectrometry, and metastable reaction monitoring mass spectrometry to examine the type and origin of the disseminated organic matter, thermal maturity, and depositional environments of the Miocene–Pliocene sediments in this basin. Results show fair–very good organic content for the Miocene Sidi Salem and Abu Madi rock samples compared to the Pliocene Kafr El Sheikh and El Wastani samples that generally show low TOC with fair–good organic content. The low organic content recoded in the Pliocene rocks may be due to high rates of sedimentation associated with clastic dilution and microbial degradation, typical of deltaic environments. In general, kerogen in the samples has high proportions of gas or non-generating and recycled organic materials and is dominantly of fluvial or deltaic origin. The analyzed source rocks exhibit a wide range of organic matter quality varying from Type-III, Type-II/III, to Type-IV kerogens. The molecular findings suggest mixed-source input from planktonic–bacterial and land plants with significant algal contributions to the source rock facies. This is based on high C30 24-n-propylcholestanes, C27 cholestanes and C29 stigmastanes, moderate tetracyclic polyprenoid (TPP) ratios, and low oleanane and gammacerane. The most striking biomarkers are high bicadinane and 24-norcholestane ratios. Local differences related to facies variation in the source depositional environment result in significant variations in biomarker characteristics. Pyrolysis Tmax, sterane isomerizations %20S and %ββ, moretane/hopane ratios, and C32 homohopane %22S suggest immature to maturity near the beginning of the oil window. Five genetic families were identified among the extract samples using hierarchical cluster analysis (HCA) and principal component analysis (PCA) based on 15 source-related biomarker ratios. Some of these families are questionable because of low TOC, significant differences in maturity between the extracts and condensates, and possible contamination by diesel additive. However, five rocks from the Miocene Sidi Salem Formation contain elevated TOC (1.22–2.25 wt%) and HI (366–458 mg HC/g TOC) and the extracts show no evidence of significant contamination in the biomarker range of molecular weight and good correlation with four WDDM-14 (2739, 2854, 2804, and 2870 m) oils in the offshore Rosetta and Abu Qir oilfields.