Biomarker Indicators of Source and Depositional Environment for the Organic Matters within Barsarin Formation (Upper Jurassic) in Kirkuk and Taq Taq Oil Fields, Northern Iraq

The GC, GC/MS analysis for twelve bitumen extracts from Barsarin Formation in the wells K-109 and Tq-1 (Kirkuk and Taq Taq oil fields respectively) and for two oil samples from K-156 and Tq-1 wells revealed a euxinic, carbonate marine to mix environment for the initial organic matters. Pr / Ph, Pr / nC 17 , and Ph / nC 18 ratios indicated also a marine source organic matters deposited under reduced condition with a less effect of biodegradation and mature stage of occurrence. The diasteranes / steranes ratio showed generally low ratios (0.27 – 0.56) as in most marine carbonate sources, while Algal-bacterial organic matter contribution for the extracts and the oil samples suggested depending on the predominance of C 23 tricyclic terpane relative over the C 19 tricyclic terpane and the C 23 tricyclic terpane greater than C 24 tricyclic terpane. The gammacerane Index indicated a low salinity environment of deposition for the initial organic matters existed in the analyzed samples.


Introduction
The Barsarin Formation was first described by Wetzel (1950), in Bellen, (1959). The type locality and type section of Barsarin Formation located in Barsarin, Rowanduz district, north east Iraq. The formation at the type section comprises of limestones and dolomitic limestone (laminated, some fluffy-textured, locally cherty) and argillaceous, brecciated, and contorted beds. The formation is believed to be-at least partly of lagoonal evaporitic origin, indicated partly by the presence of anhydrites and oolitic in layers in some sections, partly by ubiquitous brecciated and crumbled beds (Buady, 1980). The Formation expected to be a basin floor evaporite facies by Sadooni (1997). A Kimmeridgian-Early Tithonian Age for the Formation is based on its stratigraphic position below the Chia Gara Formation of the Late-Tithonian Age and above the Kimmerdigian-Oxfordian Naokelekan Formation (Jassim & Buday, 2006in Jassim & Goff, 2006.

The Analyzed Samples
Twelve extracts from Barsarin Formation has been chosen from the two subsurface sections of K-109 well from Kirkuk oil field and Tq-1 well from Taq Taq oil field, in addition to two oil samples from K-156 and Tq-1 wells to be analyzed in Baseline Resolution, Inc. (Analytical Laboratories in Huston, USA) by GC and GC/MS instruments ( fig. 1, table 1). The thickness of Barsarin Formation in Kirkuk reached to about 283m and in Taq Taq about 75m. The GC analysis for the extracts from both sections show different degrees of contamination which expected to be caused by oil base mud during drilling. The degree of the contamination varied among the samples and affected the C 15+ parameters. The three extracts from K-109 (3104, 3164, and 3364m) and one from Tq-1 section (3200m) were the least contaminated and their obtained ratios seem largely to be original or close to original. The biomarker data obtained from the saturate GC/MS analysis of the eight source rock extracts (five from K-109 and three from Tq-1) and the two oil samples show no affect of contamination from the oil base mud (Appendix).

Depositional Environment and Source Related Biomarkers
Biological markers or biomarkers are molecular fossils, (originated from formerly living organisms), and are complex organic compounds composed of carbon, hydrogen, and other elements. They occur in sediments, rocks, and crude oils and show little or no change in structure from their parent organic molecules in living organisms (Peters et al., 2005) and their carbin skeletons can be traced back to living organisms (Hunt, 1996).
The quality of information provided by the biomarkers or geochemical fossils in terms of depositional environment depends on three factors (as mentioned by Tissot and Welte, 1984):-1-Their state of conservation, which may or may not allow one to link them to their biochemical precursor molecule. 2-The distribution of the biochemical precursor (parent molecule) in the present animal and /or plant kingdom.

3-
The assumption that the distribution was comparable in ancient organisms.
The utility of biomarkers as indicators of depositional environments arises from the fact that certain types of compounds are associated with organisms or plants that grow in specific types of depositional environments (Philp, 2004). Broocks et al. (1969) in Philp, (2004) noted the presence of the regular isoprenoids pristane (Pr) and phytane (Ph) in crude oils and coal extracts. This led Powell and Mickidy, (1973) in Philp, (2004) to propose a mechanism for the production of relatively high concentrations of pristine in oxic type environments and high concentration of phytane in reducing type environments. Thus, the Pr/Ph ratio evolved as an indicator of the oxicity of the depositional environment.
Because some biomarkers pointed to specific taxa, they can also act as indicators of specific habitats. Fresh water environments are often indicated by the presence of biomarkers of typical fresh water organisms such as Botryococus braunii. Lacustrine conditions are often indicated by the predominance of algal steroids (Chen & Summons, 2002in Brocks & Summons, 2004. Hypersaline lakes and ponds often develop anoxic conditions if saline deep water is covered with water of lower density. Sedimentary rocks that were deposited under these conditions often contain high relative concentrations of gammacerane, which is a biomarker generally associated with water column stratification (Sinninghe Damste et al., 1995in Brooks & Summons, 2004.
Rearranged steranes (diasteranes) are relatively more abundant in clastic sediments than in carbonates (van Kaam-Peters et al., 1998in Brooks & Summons, 2004. Hopanoide appear to be similarly affected so that diahopanes and neohopanes are relatively more prominent in bitumen and oils derived from shales as opposed carbonates (Peters & Moldowan, 1993in Brooks & Summons, 2004. However increase in thermal maturity is also a key factor in the conversion of biomarkers to their rearranged forms.
Carbonate-dominated sediments tend to be deposited in low latitude environments and therefore biomarkers for organisms that preferentially colonize warm water tend to be important signatures in these sediments.

Pristane/Phytane:
The abundant source of pristane (C 19 ) and phytane (C 20 ) is the phytal side chain of chlorophyll (a) in phototrophic organisms and bacteriochlorophyll (a) and (b) in purple sulphur bacteria (Mc Kirdy, 1973in Peters et al., 2005. Reducing or anoxic conditions in sediments promote cleavage of the phytal side chain to yield phytol, which undergoes reduction to dehydrophytal and phytane. Oxic conditions promote the competing conversion of phytol to pristine by oxidation of phytol to phytenic acid, decarboxylation to pristene and then reduction to pristane (Peters et al., 2005). Didyk et al. (1978) in Peters et al. (2005) interpreted the redox conditions of the source rock depositional environment for crude oil based on a model for the origin of pristane (Pr) and (Ph) phytane. According to many authors, Pr/Ph less than 1 in crude oil indicates anoxic source rock deposition, particularly when accompanied by high porphyrin and sulphur contents, while Pr/Ph more than 1 indicates oxic deposition. Pr/Ph is commonly applied because Pr and Ph are measured easily using gas chromatography.
In summary, the Pr/Ph ratios in the range of 0.8-3.0 are interpreted to indicate specific paleoenvironmental conditions without corroborating data. Pr/Ph more than 3.0 indicates terrigenous plant input deposited under oxic to suboxic conditions, while Pr/Ph less than 0.8 indicates saline to hypersaline conditions associated with evaporate and carbonate deposition (Peters et al., 2005).
The ratio Pr/nC17 is useful for differentiating organic matter from swamp environment (more than 0.1) from those formed under marine environment (less than 0.5), but this ratio is affect by maturity, as well as biodegradation (Osuji & Antia, 2005). According to Elvsborg et al., (1985), Cohen and Dunn, (1987) both in Sletten, (2003) Pr/Ph ratios typical for type II marine shales are 0.6-1.6.
An oil accumulation have pristane / phytane ratio more than 3.0, indicative of a predominantly non marine source from terrestrial organic matter. Furthermore, the progressive change from waxy to light oil has been interpreted to indicate an increased contribution of bacterial or algal source inputs to the lighter oil (Powel et al., 1989in Boreham & Hill, 1998). An alternative interpretation is that the original source input, although dominated from terrestrial organic matter, has been masked by secondary alteration processes.
The ratios of the Pr / Ph for the four uncontaminated extracts mentioned above in this study were generally low and ranged between 0.34 and 0.61 (less than 0.8) (table 2) indicating anoxic, reduced marine carbonate depositional environment.
The ratio of Pr / nC17 for the same four extracts ranged between 0.15 and 0.29 (less than 0.5) indicating marine environment of deposition also.
The cross plot of Pr / nC17 versus Ph / nC18 for the whole analyzed samples ( fig. 2) shows clear marine source organic matters deposited under reduced condition with a less effect of biodegradation and mature stage of occurrence. and 0.87 for Tq-1 oil) and those of the extracts may be interpreted as those two oils generated from additional marine sources of organic matters (generally of Jurassic age) and not only from Barsarine source rocks.

Diasteranes/Steranes:
Diasteranes / steranes ratios are commonly used to distinguish petroleum from carbonate versus clastic source rocks (Mello et at., 1988(Mello et at., in peters et al., 2002. Low diasteranes / steranes ratios (mass/charge 217) in oils indicate anoxic clay-poor or carbonate source rocks. During diagenesis of these carbonate sediments, bacterial activity provides bicarbonate and ammonium ions, resulting in increased water alkalinity. Under these conditions of high pH and low Eh, calcite tends to precipitate and organic matter preservation is improved (Peters et al., 2005). High diasteranes / steranes ratios are typical of petroleum derived from clay-rich source rocks and in some crude oils can result from high thermal maturity (Seifert & Moldowan, 1979in Peters et al., 2005 and/or heavy biodegradation (Peters et al., 2005).

Figure
(2): Pr/nC17 versus Ph/nC18 cross plot, from which a mature, marine source organic matter can be detect for the analyzed samples.

Ph / n-C18
At high levels of thermal maturity, rearrangement of steroids to diasterane precursors may become possible, even without clays, due to hydrogen-exchange reactions, which are enhanced by the presence of water (Van Kaam- Peters et al., in Peters et al., 2005). Alternatively, diasteranes simply may more stable and survive thermal degradation better than steranes. The diasteranes/steranes ratio is useful for distinguishing sourcerock depositional conditions only when samples show comparable levels of thermal maturity (Peters et al., 2005).
Heavy biodegradation can result in selective destruction of steranes relative to diasteranes. However, it's possible that non-biodegraded oil might mix with heavily biodegraded oil showing a much higher diasteranes/steranes ratio. In such cases, only careful quantitative assessment of each biodegradation-sensitive parameter can led to the correct interpretation (Peters et al., 2005).
The diasteranes / steranes ratio for the analyzed extracts and oil samples show generally low ratios (0.27 -0.56) (table 3) indicating anoxic carbonate source rocks.
There is a positive correlation between Pr / (Pr+Ph) and C 27 diasteranes/ (diasteranes+steranes) that were controlled by the depositional environments (Moldowan et al. 1994in Peters et al., 2005. Pr / (Pr+Ph) increases with clay content, as measured by increasing diasteranes, which parallels oxidative strength (Eh) of the water column during deposition. Depending on the relation between Pr / (Pr+Ph) and C 27 diasteranes/ (diasteranes+steranes) ( fig. 3) a slight difference in the source of the organic matters between K-109 and Tq-1 sections appeared as a contribution from anoxic shale source seems to be occurred in the section K-109 while anoxic carbonate source is clear in Tq-1.
The cross plot of C 27 / C 29 diasterane versus C 27 / C 29 sterane used to identify the source of the analyzed extracts and oil samples in this study ( fig. 4).The result of this cross plot indicated a mixed origin of organic matters, and all the extracts and the two oil samples have generally C 27 / C 29 diasterane values larger than C 27 / C 29 sterane (table 4).

C 27 , C 28 , and C 29 Steranes Ternary Plot:
Haung and Meinschein (1979) in Peters et al., (2005) proposed that the distribution of C 27 , C 28 , and C 29 sterol homologs on a ternary diagram might be used to differentiate ecosystems.

Journal of Kirkuk University -Scientific Studies , vol.3, No.1,2008
The ternary plot of the relative amounts of C 27 , C 28 , and C 29 steranes for the analyzed samples (table 4)

C 30 Sterane Index [C 30 / (C 27 -C 30 ) Steranes]:
The C 30 sterane ratios generally increase with marine versus terrigenous organic matter input to the source rock (Peters et al., 2005). The presence of C 30 4-desmethysteranes in crude oil is the most powerful means in order to identify input of marine organic matter to the source rock (Peters et al., 1986in Peters et al., 2005. These C 30 steranes are identified as 24-n-propylcholestanes, which originate from 24-n-propylcholeseroles (Moldowan et al., 1990in Peters et al., 2005 Ratio of C 30 / (C 27 -C 30 ) steranes against oleanane / hopane give a better assessment of marine versus terrigenous input to petroleum than either parameter alone. High oleanane and lower C 30 steranes, suggesting a deltaic source rock with strong terrigenous input. High C 30 steranes and lower oleanaes, suggesting a marine source rocks with less terrigenous input.
The generally high ratios of the C30 Sterane Index of the analyzed samples (3.12 -3.83) (table 4) is an indication for marine source of organic matters. Plotting the obtained values of C 30 / (C 27 -C 30 ) steranes versus oleanane index for the two extract samples from the depths 3165m and 3200m in Tq-1 well and the oil sample of K-156 revealed a marine source organic matters rather than terrigeneous origin figure (6).

Gammacerane:
A C 30 pentacyclic triterpane in which each ring contains six carbon atoms. Source rock deposited in stratified anoxic water columns (commonly hypersaline) and related crude oils commonly have high gammacerane indices (gammacerane / hopane). The gammacerane Index (gammacerane / hopane C31) for all analyzed samples (extracts and oil samples) show a low ratio (0.1 -0.3) (table 5) indicating a low salinity environment of deposition for the initial organic matters.
Higher salinity is typically accompanied by density stratification and reduced oxygen content in bottom waters (i.e. lower Eh), which results in lower Pr / Ph. Figure (7) shows the relationship between the gammacerane index and Pr / Ph ratio for the analyzed oil and extract samples. The cross plot indicates a marine environment of deposition and it is comparable with the results of the diagram of peters et al., (2005).

Terpanes:
The biomarker compounds referred to as terpanes are mainly derived from bacterial (prokaryotic) membrane lipids (Mukhopadhyah, 2004). These ratios help to identify extracts and crude oils derived from carbonate source rocks. Generally, oils from carbonate source rocks can be distinguished by high C 22 /C 21 tricyclic terpane and low C 24 / 23 tricyclic terpane (Peters et al., 2005). The plot of the analyzed extracts and oil samples (specially the oil sample from K-156) (table 5) ( fig.8) shows a great similarity with the analyzed extracts and oil samples of carbonate origin worldwide by Geo Mark Research Inc. (2000) in Peters et al., (2005) and that relating to their included ratios of tricyclic terpanes C 22 /C 21 versus tricyclic terpanes C 24 /C 23 . In the terpane distribution (m/z 191), the predominantly algal-bacterial organic matter contribution for the extracts and the oil samples can be suggested from the C19 -C25 tricyclic terpanes distribution by the predominance of C23 tricyclic terpane relative over the C19 tricyclic terpane and the C23 tricyclic terpane greater than C24 tricyclic terpane. The C19 / C23 tricyclic terpane ratio is thus very low (extracts: 0-0.33; oils: 0.21 and 0.60) and the C23 / C24 tricyclic terpane ratio is greater than 1 (extracts: 1.59-1.83; oils: 2.16 and 2.71). In the pentacyclic terpanes, the high C24 tetracyclic / C26 tricyclics ratio (extracts: 1.57 -4.18; oils: 3.94 and 4.04), the high norhopane / hopane ratio (extracts: 1.08 -1.44; oils: 1.12 and 1.43), and the high C35 / C34 hopane ratio (extracts: 0.88 -1.04; oils: 1.01 and 1.19) (table 5) indicate marine carbonate facies and anoxic depositional condition for the source rock of the extracts and the oils.
Figure (9) shows the relationship between norhopane / hopane (C29H/C30H) and C35 / C34 hopane ratios for the analyzed extracts and oil samples. From the cross plot a carbonate content and anoxic condition dominant is clear. A high hopane/Sterane ratio can indicate terrestrial input, while low ratios are typical for marine derived petroleum (Sletten, 2003). Based on this, all the analyzed samples as appears in the cross plot between Pr/Ph ratio and hopane/ sterane ratio ( fig.10) indicate a marine source of organic matters.

Conclusions
 The GC/MS analysis for extracts from selected samples from Barsarin Formation in the two sections of K-109 and Tq-1 indicated a euxinic, marine (to mix) carbonate source of deposition. The same source also supposed for the two oil samples taken from K-156 and Tq-1 wells.  Pr / Ph, Pr / nC17, and Ph / nC18 ratios indicates marine source organic matters deposited under reduced condition with a less effect of biodegradation and mature stage of occurrence.  The diasteranes / steranes ratio for the analyzed extracts and oil samples show generally low ratios (0.27 -0.56) indicating anoxic carbonate source rocks.  Algal-bacterial organic matter contribution for the extracts and the oil samples can be suggested.  The gammacerane Index indicated a low salinity environment of deposition for the initial organic matters existed in the analyzed samples of both extracts and oil samples.