Palynofacies Analysis and Hydrocarbon Generation Potential of the Tithonian-Beriassian Chia Gara Formation from Selected Wells in Ajil, Hamrin, and Tikrit Oilfields, Northern Iraq

The recovered sedimentary organic matter from the Tithonian-Berriasian Chia Gara Formation was studied from three selected wells of Ajil-12 (Aj-12), Hamrin-1 (Hr-1), and Tikrit-3 (Tk-3) in Northern Iraq. Amorphous organic matters appeared to be the dominant components of the organic matters within the formation with more than 93%, whereas phytoclasts and palynomorphs comprised only few percentages. As no intensive variations observed in the percentages of the determined organic matters along the studied Chia Gara Formation except a slight increase in phytoclasts and palynomorphs in its uppermost part, therefore the formation considered to be representing only one mainpalynofacies type that subdivided into two secondary palynofacies. Using Tyson's APP ternary, the formation appeared to be deposited in a distal suboxic-anoxic basin. As source rock, the formation in the studied locations contains moderate to very good quantity of organic matters which compose mainly of type II and Type III kerogen (separately or mixed). The whole formation has the ability to generate oil or gas except the upper part of the formation in the location of the well Aj-12 in which less than the lower limit of the required S2 values for considering as source rock is recorded.


Introduction
Sedimentary Organic Matter (SOM) can be defined as biogenic material that is preserved in sedimentary rocks. The organic matter is composed of both insoluble materials known as kerogen, and soluble bitumen and oil [1]. Both the composition and distribution of the organic matter are controlled by ecological conditions and sedimentological processes in the depositional environment, while microbial, physical and biogeochemical processes in sediments influence its abundance [2].
Among the importance of studying origin and character of SOM in sediments is to determine the amount and type of hydrocarbons that may be generated from sediment during burial and thermal maturation [2]. Palynofacies as concept first introduced by [3].His definition for the term as paraphrased by [2] is the palynological study of the total assemblage of particulate organic matter contained in a sediment following removal of the sediment matrix by HCl and HF [2] extended the definition of the palynofaciesto be"a body of sediment containing a distinctive assemblage of palynological organic matter thought to reflect a specific set of environmental conditions, or to be associated with a characteristic range of hydrocarbon -generating potential".
On the other hand, palynofacies analysis refers to the process that involves identification of organic matter constituents, calculating their relative and absolute abundances, and determining their size and degree of preservation [4].
In this study, an attempt done to characterize one of the effective source rocks within the Iraqi geological column known as Chia Gara Formation and that through analyzing the percentages of its sedimentary organic matter's components. Chia Gara Formation generally consists of a succession of thin beds of organic matter rich limestone, argillaceous limestone, and shales with rich ammonite faunas and diverse species of foraminifera, radiolarian, and ostracodes [5,6,7]. Determining the nature of the depositional environment, dominant types of the organic matters, and ability of the formation to generate hydrocarbons were among the main aims of this study.

Sampling and Methodology
The

Classifications of Sedimentary Organic Matters
The palynological organic matter classified according to [2] and [10] into two major categories: Structureless or Structured palynological organic matter: Structureless organic matter is defined as an organic matter that lacks a definite internal structure when observed using light microscopy, lacks a distinct and recognizable outline, and which does not infer its biological affinity. Accordingly, structureless organic matter includesmaterials such as Amorphous Organic Matter (AOM), resin, and humic gel.
Structured organic matter is made of discrete and recognizable individuals or colonial entities (i.e. palynomorphs) and plant or animal fragments (i.e. phytoclasts andzooclasts) that demonstrate their biological affinities.
AOM is a heterogeneous, yellow to grey colored material that is made of amorphous materials and is mainly produced by biodegradation of algal phytoplankton blooms, derived from zooplankton faecal pellets, or derived from biodegradation of cyanobacteria and thiobacteria [2].AOM is considered the major contributing component to structureless organic matter in ancient marine and lacustrine sediments [2]. The high relative or absolute abundances of AOMusually associated with sediments beneath upwelling water masseswas taken to indicate bottom water of low (dysoxic) oxygen concentrations [11,12].
Palynomorphs are either of botanical or zoological affinities, whereas phytoclast particles with coherent, angular to irregular outlines that may show some internal structures can be attributed at least to a type of larger plant (phytoclasts) or animal (zooclasts) debris [2]. In this study, the identified and examined organic matter components subdividedinto three main classes namely Amorphous Organic Matter (AOM), Phytoclasts, and Palynomorphs. AOM included all the structurless organic matters (without determining their sources). The phytoclasts included opaque organic materials, wood debris, and cuticles, whereas the palynomorphs represented all kinds of fossilized organic walled organisms including foraminiferal test linings. Accordingly, the categories used in this study are very similar to those used by [2] as shown in Table 1  Table 1: Categories used in this study to identify the organic matter components with their representative sources and constituents (after [2]). Table 2

Palynofacies types in the studied Chia Gara Formation
As appeared in the Table 2 Sample points plotted on the APP ternary diagram for Chia Gara Formation in the studied three sectionsindicated (as expected)only one main palynofacies typewhich represent the deposition in a distal suboxic-anoxic basin (Field IX) Fig. 5. According to [2], the field IX of his ternary has the following properties:  The dominance of amorphous organic matter assemblages.
 Low abundance of palynomorphs partly due to masking.
 Frequently rich of alginate.
 Deep basin or stratified shelf sea deposits, especially sediment starved basins.  The organic matters are expected to be oil prone and the type II kerogen is more dominant than type I.

Hydrocarbon Generation Potential of Chia Gara Formation
To understand the hydrocarbon generation potentiality of the identified palynofacies of Chia Gara Formation; pyrolysis analysis done for the selected 26 rock samples from wells Aj-12 and Tk-3 wells. In addition, pyrolysis data from [8] also used for the well Hr-1.  Palynomorphs for the studied Chia Gara Formation on the ternary proposed by [16] The value of S2 in the process of pyrolysis represents the amount of hydrocarbons generated from the kerogene by thermal cracking at temperatures between 400 and

Conclusions
The following are summarization for the main conclusions of the study:  The organic matters within Chia Gara Formation in the studied sections of the wells Aj-12, Hr-1, and Tk-3 compose of high percentages of AOM with only few percentages of phytoclasts and palynomorphs. Therefore, only one primary palynofacies type can be distinguished in the formation which can be subdivided into two secondary palynofacies due to a slight increase in the percentages of the phytoclasts and palynomorphs at the upper part of the formation.   AOM of different shapes and thicknesses can be found in Chia Gara Formation which believed to be of mixed sources (marine and terrestrial). This true also with the palynomorphs in Chia Gara Formation which compose mainly of marine dinoflagellates and terrestrial spores and pollen grains.
 The formation contains moderate to very good quantity of organic matters. As noticed in the wells Aj-12 and Tk-3, the lowermost part of the formation appears to be the richest of organic matters and the upper part the lowest.
 According to the palynofacies analysis results, the organic matters within the formation consists mainly of type II kerogen with contribution from type I kerogen also. This is not totally coincides with the results of the pyrolysis analysis which indicated clearly to contribution of type III kerogen also especially in the well Hr-1.
 The hydrocarbon potentiality of the formation as source rock can be considered as moderate to very good except the upper part of the formation in the studied section of Aj-12.