Integrated organic geochemistry with palynofacies analysis of the Chia Gara Formation in the Kirkuk and Hamrin oilfields, North Iraq

The idea of the present work was to combine Rock-Eval pyrolysis and palynofacies analysis in an integrated approach to study 51 source rock cores and cuttings samples collected from the middle TithonianBerriasian Chia Gara Formation, in well K-109, Kirkuk Oilfield and well Hr-1 drilled in the Hamrin Oilfield, north of Iraq to evaluate quantitatively and qualitatively the nature of sedimentary organic matter, generation potential and assess thermal maturity of the studied formation in both wells. Rock-Eval pyrolysis and palynofacies data indicate that the Chia Gara Formation in well K-109, Kirkuk Oilfield has kerogen type III/II and II/I and the amorphous organic matter AOM is dense and amber brown to very dark brown in color assigned to type C and D which reflects mature to overmature stage (gas-oil prone). On the other hand, the source rock samples of the Chia Gara Formation encountered in well Hr-1, Hamrin Oilfield has kerogen type II/III and II/I and the AOM is dense rounded, yellow to yellowish brown typeD, mature and is oil-gas prone. The regional tectonic movements and thermal history may cause the diversity in kerogen type and the subtle color variation in the AOM from the Chia Gara Formation in the Kirkuk Field in the north to Hamrin Field in the south.


INTRODUCTION
Iraq is a long-standing petroleum-producing country in the Middle East and a founding member of the cartel of the Organization of Petroleum Exporting Countries (OPEC). The current proven reserve of 133 billion barrels of oil and 110 trillion cubic feet of gas is recovered from stacked pay-zones along the stratigraphic column [1,2]. Major basins in the Kirkuk University Journal /Scientific Studies (KUJSS) Volume  world have multiple source rock horizons. The northern part of Iraq is characterized by more than one source rock belonging to Sargelu, Najmah, Chia Gara and Balambo [3]. These source rocks feed the Cretaceous-Tertiary reservoirs [1,2]. Oil and gas reservoirs are dispersed through the Cretaceous and Tertiary rocks [4], sealed by the Middle Miocene Fatha Formation, that extends from the north to the middle of Iraq and includes mainly bedded anhydrite with shale intercalations [5]. [6] and [3] acknowledged that the principal source rocks were deposited during the Late Jurassic to Early-Middle Cretaceous, and the weight of evidence suggests that oil from these two sources are responsible for charging most of the Iraqi reservoirs.

GEOLOGIC BACKGROUND
The study area Fig. ( [4,7,8,9]. [8] and [9] recognized five tectonophysiographic zones: three in the unstable shelf, and two in the stable shelf. These tectonic zones include 1) the Thrust Zone, 2) the Folded Zone, 3) the Mesopotamian Basin, 4) The Salman Zone, and 5) The Rutbah-Jezira Zone. The study area is related to low Folded/Foothill Zone of the Folded Zone in which mainly Cenozoic rocks are exposed [10,11].
The Kirkuk-Hamrin folded subzone, to the southeast of the Greater Zab River, contains long, unbroken anticlines [11]. The Kirkuk anticline contains three separate domes simple with few thrusts [11]; its geometry suggests that it is a forced fold above the suspected Zagros Foredeep Fault Zone, which is thought to have been reactivated in the Pliocene to Formation Fig. (2). The type section of the Chia Gara Formation is located at the Chia Gara anticline, south of Amadiya town in the strongly Folded Zone of north Iraq [5].
Lithologically, the formation is uniform throughout Iraq, consisting of two basic lithofacies types: thinly bedded limestone and calcareous shale in the lower part of the section, followed upward by marly limestone and marl [6].

Samples and analytical procedures
Fifty-one core and cuttings samples recovered from the Chia Gara Formation in Kirkuk and Hamrin oilfields, northern Iraq, have been subjected to total organic carbon (TOC) and Rock-Eval pyrolysis analyses, which were conducted in StratoChem Laboratory, New Maadi, Cairo, Egypt. These analyses were carried out using a Rock-Eval 6 pyroanalyzer equipped with a TOC module. The samples were heated from 300°C (hold time 3 min) to 850°C (hold time 5 min) at 25°C/min for oxidation.
For palynofacies analysis, the samples were prepared palynologically using standard palynological maceration techniques [13] adopted at the Geology Department, Faculty of Science, Mansoura University, Egypt.

RESULTS AND DISCUSSION
Analytical data on rock samples using TOC/Rock-Eval are summarized in Table (1) and Figs. (3) and (4) , and palynofacies data in Table (2) , and Fig. (5) and Plate 1.
The cross plot of TOC and Rock-Eval S 2 are excellent approach for evaluating the organic richness of a source rock [14,15,16], whereas the cross plot of hydrogen index (HI) versus oxygen index (OI) is used for determining the kerogen type [17].
As displayed in Fig. ( from organic petrography and biomarkers are the most common parameters for assessing sedimentary organic matter thermal maturity [16,17,18]. In this study, we use T max in order to estimate the thermal maturity level of the Chia Gara rock samples Gara source rock may result from regional tectonic regimes and thermal history from one basin to another.
The palynofacies characteristics of the major potential source rocks in northern Iraq have been previously investigated for the Jurassic-Early Cretaceous Chia Gara Formation by [19,20,21,22].
In the present work, the palynofacies analysis of the Tithonian-Berriasian Chia Gara  In this work, we use ternary diagrams for illustrating our data due to their good potential in showing a clear separation among various groups of samples or assemblages [24]. The AOMphytoclast-palynomorph APP [25] is applied in this work. He uses it to characterize kerogen assemblages, pick out the differences in relative proximity to terrestrial organic matter sources, kerogen transport paths and the redox status of the depositional subenvironments that control AOM preservation.Data plotted on the APP ternary diagram indicated one main palynofacies type Fig. (5) , Table (2) . The palynofacies samples representing the Chia Gara Formation lie mainly within the field IX, which reflects distal-suboxic-anoxic basin, based on the high content of AOM that dilutes all other organic particles, and hence classified as type-II>>-I kerogen, highly oil-prone [24].

Fifty-one cores and cuttings samples retrieved from the Tithonian-Berriasian Chia Gara
Formation succession in Kirkuk and Hamrin oilfields from northern Iraq, were studied by total organic carbon (TOC), Rock-Eval pyrolysis and palynofacies analyses. The following key points can be drawn: 1. Rock-Eval pyrolysis data indicated a wide variation of source richness and quality, but all contain varying quantities of Type-II (oil-prone) and Type-III (gas-prone).

2.
The thermal maturity of the studied rock samples ranges from early to peak oil window based on the Rock-Eval T max data.

Palynofacies analysis using the transmitted light microscopy enabled us to locate the
Tithonian-Berriasian Chia Gara Formation palynofacies mainly within the field IX, which reflects distal-suboxic-anoxic basin, and hence classified as type-II-I, oil-prone [24].

4.
In the present investigation, it seems that results gained from optically-predicted methods indicate that kerogen type is assumed to belong to type-II-I, oil-prone, and actual pyrolysis data confirm a mixed type-II-III and type-III, oil-prone to oil-and gas-prone.  well Hr-1,Cuttings _3155 m_O37.