Subsurface geochemical and mineralogical evaluation for unconventional “shale” oil play of the Bahloul Formation (Cenomanian-Turonian) in the Sahel Basin, Eastern Tunisia

Abstract

This paper evaluates the Cenomanian-Turonian Bahloul Formation as a new target for an unconventional shale oil play in the Sahel Basin, eastern Tunisia to increase the oil supply. A detailed geochemical (Rock-Eval pyrolysis and biomarkers) and mineralogical study was carried out on 44 drilled cutting samples of the Bahloul Formation from three wells (M-1, A-1, and T-2). The geochemical investigation highlighted variable total organic carbon (TOC) concentrations ranging from 0.20 to 6.65 wt.%, with the highest values recorded in the samples from the M-1 well (avg. 2.76 wt.%) and A-1 well (avg. 1.68 wt.%). The lowest TOC values are observed in the samples from the T-2 well (avg. 0.65 wt.% TOC). These organic matter (OM) contents reflect favorable conditions for the accumulation and preservation of OM controlled by both an oxygen minimum zone (OMZ), and restricted circulation in a “semi-silled” basin. This basin is marked by the presence of subsiding zones and paleohighs (Triassic salt domes, and horsts). Within the subsiding basin, the preserved OM contents are primarily marine OM (type II) with a minor contribution of terrestrial (type III) and/or bacterial/algal (type I) material. The nature of the OM (type II) and the mean hydrocarbon generation potential (HGP) of 9.26 kg HC/ton rock render Bahloul Formation an oil-prone source rock. Additionally, the thermal maturity of the OM preserved in this formation varies between late diagenesis (avg. Tmax = 427°C in M-1 well) and the early oil window (avg. Tmax = 438°C and 435°C, in A-1 and T-2, respectively). The average transformation ratio (TR) in wells A-1 and T-2 is estimated at 40%. Within these wells, the oil saturation index (OSI) reveals the presence of three “oil cross-over” intervals attributed to the high degree of oil source rock saturation (OSI > 100%) and the accumulation of hydrocarbons (HC). These “oil cross-overs” resulted in primary or secondary oil migration into Bahloul source rock. The primary oil migration is considered to be facilitated by the development of organic and nonorganic porosities and the onset of interconnected, continuous, oil-wet filets. On the other hand, the secondary migration seems to be favored by an intensely fracturing network in connection with open fracture systems and faults achieved during the basin’s geodynamic history. These “oil cross-overs” determine the presence of three target intervals in each of A-1 and T-2 wells. In A-1 well, these targets have a significant thickness (from 3 to 6m) but in T-2 well they are only 1–2m thick. The in situ oil reserves are estimated at 3.109 tons HC in the areas where OM is mature (7,000 km2), with a free HC quantity (avg. S1 = 20 % of HGP) of 6.108 tons HC. These volumes of HC can be liberated by hydraulic fracturing. The Bahloul Formation is ideal for hydraulic fracturing owing to its composition (limestone: 50-77% CaCO3 and a minor detrital fraction) and its high brittleness index (BI = 0.83 to 0.90).