The Petroleum System of Zubair Formation in Zubair Subzone, Southern Iraq

The Lower Cretaceous Zubair Formation is the most important reservoir in the south of Iraq and adjacent countries, as well as the shale intervals, may be the possible source rock. The parameters of the possible source rock within Zubair Formation are accepted, regarding quantity, quality, and thermal maturity. From the total organic carbon values standpoint, shale intervals of Zubair Formation are generally good source rocks, except in the lower parts. While the kerogen types are generally type III gas-oil prone, mix type II/III, and type II oil-prone. The main type of organic matters was amorphous organic matters, which is principally related to oil-prone source rock. The Rock-Eval Pyrolysis shows that the organic geochemical properties presented an effective and/or potential source rock depending on the values of S1, S2, and S3 and their derivatives. The source rock has started the oil expulsion, where the S1/TOC values were mainly more than 0.2 mg HC / g TOC. The modelling explains that the Zubair Formation, wholly, has been entered the onset, partially, the peak, and rarely the end of oil generation in the studied Subzones. This conclusion depends on the calculated temperatures, geothermal gradients, and Vitrinite reflectance calibration. No.25(12) 2019 Journal of Petroleum Research & Studies (JPR&S) E58


Material and Methods:
Twenty-three cores and cuttings samples were collected from Zubair Formation and their suggested hard data source rocks from four fields in the Zubair subzone which represented by Basrah regions. These data were used to the different geochemical analysis as shown in Table (1), for Rock-Eval pyrolysis. These samples are subjected to organic geochemical analysis with pyrolysis for source rocks evaluations in Geomark Research Ltd in Houston, Texas.

Quantity of Organic Matter:
The quantity of Organic Matter (OM) is usually expressed as Total Organic Carbon TOC [9]. TOC determination can be considered as the first step to provide basic geochemical information in order to evaluate the source rocks, as well as to select a sample for more detailed analysis.
The results of measured TOC values in the Zubair Formation vary in Zubair Subzone as shown in Table (2) [10]. The TOC results show that approximately the same pattern of distribution except in the lower part, namely lower shale member, characterized by a decrease in TOC, which may indicate a terrestrial OM invasion or oxic paralic siliciclastic environments represented by delta front, delta-top set, or fluvio-deltaic environment which containing on recycling [11]. The quantity of TOC in rocks is closely related to sediment particle size. High TOC in sediments is due to the preservation and transportation of OM, not organic productivity [12]. The preservation of TOC generally increases from high-energy, which mainly coarse- grained sediments, to low energy, which characterized by fine-grained sediments, as well as the sand unit with high values of TOC refer to the interbedded shale layers of prodelta and fine-grained sand interval, which may indicate low to moderate energy environment.
These may interpret the relatively high values of TOC in the sand layers Zubair Formation.  E62 essentially related to their hydrogen content. The amount of HC which is generated and expelled from an SR increases as the atomic H/C ratio increases [9]. Thus, the most useful classification of kerogen is based on this ratio. The amorphous and algal, lipid-rich terrestrial kerogens like resinite and cutinite are oil-prone. Amorphous kerogens are the most abundant type of OM in SR. The kerogen typing is integrated by visual kerogen characteristics, as previously mentioned, and HI vs. OI and S2 vs. TOC crosses plots, as explained in Figures (2 and 3). The main values of HI in Zubair intervals in the studied area were between 50-200 mg HC/g TOC, Table (3). As a result, the kerogen type of the Zubair Formation can be considered as type III gas-oil prone, II/III oil-gas prone, II oil-prone, and less frequently of type I oil-prone as shown in Table (3).

S1 versus TOC:
A plot of S1 vs. TOC can be used to discriminate between indigenous and nonindigenous HC. Migrated or contaminants HC are indicated when S1 values are higher comparing with TOC, which have to be relatively low in the same sample. This case is represented by the values above the slanted line in Figure 4. However, the cross plots show that most of the samples are indigenous.

Thermal Maturity of Source Rocks:
The pyrolysis Tmax values in Zubair Formation are mainly up to 430 C°,  [16]. These values indicate that the OM in the units of studied formation is thermally mature with respect to HC generation, Figure

Fig. (5) The Production Index (Transformation Ratio or PI) versus calculated Ro as an indicator to maturity, which it is referred to onset of generation and expulsion in
Zubair Subzone and indicate that Zubair Formation entered the oil window.

Hydrocarbon Generation Modeling and Reservoir Properties:
Timing of the start, peak and end of HC generation (Transformation Ratio equal to 1, 50, and 99% respectively, for SR, is calculated at each unit of the Zubair Formation [19]. The beginning of HC generation from the possible SR in the Zubair Formation, the peak of HC generation, and the end are explained in Table (4).
As detailed in Table 4, in Zubair Subzone, the studied locations, mainly have reached to the peak of HC generation and some of these such as Zb_41 and Ru_4, 72 are at the end of HC production.
The immature intervals of Zubair Formation, or any other rich source rock, may be targeted as resources for "Oil Shale" industry in the near future. The term oil shale generally refers to any sedimentary rock that contains solid bituminous materials that are released as petroleum-like liquids when the rock is heated. To obtain oil from oil shale, the shale must be heated and resultant liquid must be captured. This process is called retorting, and the vessel in which retorting takes place is known as a retort [20].

Conclusions:
The following is a summary of the main conclusions that can be established from the result of the previous sections: 1. The TOC of Zubair Formation approximately ranges between 0.77-25.8 percentage, while the average values range 0.8-3.0%.
2. The principal kerogen type in Zubair Formation is the gas-oil prone Type III Kerogen with hydrogen index between 50-200 mg HC/g TOC, but oil prone Type II and I Kerogens are available at least in 26m, located mainly in the Zubair Subzone.
3. The pyrolysis parameters introduced that the Zubair Formation is entered the threshold of oil generation and expulsion, fortunately after the trap formation, which is represented by the values of S1 (more than 0.2 mg HC/g TOC), as well as the petroleum potential (more than 2.5 mg HC/g TOC), especially in the Zubair Subzone.
4. The oil of Zubair Formation is indigenous within the studied area as it has been indicated by the relation between S1 vs. TOC.
5. The maturity parameters: Tmax and Ro indicate that the Zubair Formation is mainly within the early-peak oil window.
6. The oil generation prospectivity of the Zubair Formation clearly appeared in the sandstone members. This result may be indicated in thin organic-rich marine shale beads deposited in the prodelta zone and belonged to the offshore facies.
7. The software modelling shows that all members of Zubair Formation are within the start of hydrocarbon generation, but less in the peak and rare at the end of hydrocarbon generation with respect to the transformation ratio.