Introduction

The Kurdistan Region is an oil-rich area and has become the largest oil-producing portion of north Iraq. The Late Jurassic-Early Cretaceous sediments are widespread and occur in Northern Iraq. The dataset used herein is from the Banik Village, Duhok Governorate, Kurdistan Region (Fig. 1a). The study area occupies the Northern part of the Kurdistan Region, which lies between latitudes 37° 13′ 33.4″ N and longitudes 42° 58′ 2.6″ E (Fig. 1b) and has attracted the interest of numerous researchers. The Banik Village is located at about 25 km northeast of Zakho City in the Kurdistan Region. The area is situated within the highly folded zone of Iraq (Buday 1980; Edilbi 2010). The Banik section also lies in a rugged terrain where the section of Jurassic rocks is exposed along a relatively narrow valley in which Banik Village lies (Fig. 1c). The exploration successes in the Kurdistan Region of north Iraq have attracted the attention of many petroleum researchers and explorers. However, a detailed geochemical characterisation of the Late Jurassic-Early Cretaceous sediments in the Banik Village has not been reported to date in any of the previews literature; thus, there is poor knowledge on the organic facies variation and distributions. Integrating organic and inorganic geochemistry provides a multi-scale view and increases knowledge about the organic matter type, source inputs, depositional environment conditions, mineralogy and chemical composition of sedimentary strata. The present study focuses on the geochemical characteristics of the Late Jurassic-Early Cretaceous limestone sediments from the Banik Village in the Kurdistan Region, Northern Iraq, so as to provide an overview on the origin of organic matter and depositional environment conditions. The organic and inorganic geochemical data involved total sulfur (TS) contents, bitumen extractions, biomarker distributions and major trace element composition. Biomarker distribution is widely applied and successfully used to interpret deposit environment conditions and the thermal maturity for organic matter preserved in sediments (Peters and Moldowan 1993; Peters et al. 2005). The biomarker investigations can also give more detailed information needed to answer exploration questions on source input and conditions of depositional environment of organic matter. Thus, we report herein results from an investigation on limestone sediments using detailed biomarker characteristics, coupled with inorganic geochemical elements.

Fig. 1
figure 1

a Location map for the northeast Arabian Peninsula in Iraq, which shows the Zagross Fold Belt with oil and gas field locations in the Iraqi Kurdistan (compiled and modified using the map from Al-Ameri and Zumberge 2012). b Location map of the studied section from Banik Village in the Kurdistan Region. c Satellite image of area around Banik Village (modified after Edilbi 2010)

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Sampling and experimental methods

A total of ten outcrop limestone samples were collected from Late Jurassic-Early Cretaceous sequence in the Banik Village within the Kurdistan Region from Northern Iraq (Fig. 2). Since weathering is always a factor of concern for geochemical analyses of outcrop sediments, the weathered rock surfaces were removed by digging in each sampling point. The samples then were scrubbed and exhaustively cleaned with distilled water to remove traces of superficial dirt and plant growth, then dried at 35 °C for 12 h. Most of organic and inorganic geochemical analyses were performed on limestone samples and conducted at the Geochemistry Laboratories of the Department of Geology in the University of Malaya.

Fig. 2
figure 2

Sedimentary log of the studied Jurassic-Early Cretaceous limestone sediments from Banik Village in the Kurdistan Region, Northern Iraq

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Extraction and fractionation

The whole limestone samples were extracted (after fine grinding) with a mixture of dichloromethane (DCM) and methanol (CH3OH) (93:7) for 72 h using a Soxhlet apparatus. The bitumen fractions of the limestone extracts were fractionated using liquid column chromatography on silica gel below alumina. Elution with petroleum ether produced the aliphatic hydrocarbon fraction, elution with DCM produced the aromatic hydrocarbon fraction and elution with CH3OH produced the polar compounds. Aliphatic fractions were dissolved in hexane and analysed by a gas chromatography–mass spectrometry (GC-MS) instrument.

Gas chromatography–mass spectrometry

GC-MS was carried out using a HP 5975B MSD system in full scan (50–550 amu) modes. The GC was HP-5MS column and temperature programmed from 40 to 300 °C at a rate of 4 °C/min, and then held for 30 min at 300 °C. GC-MS analysis was performed on a HP 5975B MSD mass spectrometer with a gas chromatograph attached directly to the ion source (70-eV ionisation voltage, 100-mA filament emission current, 230 °C interface temperature). The selected ion monitoring capabilities of the data acquisition system permitted specific ions to be monitored, such as n-alkanes (TIC), terpanes and hopanes (m/z 191), and steranes (m/z 217) (Amijaya et al., 2006; Korkmaz and Kara Gülbay 2007; Hakimi et al. 2011). Relative abundances of triterpenes and steranes were calculated by measuring peak heights in the m/z 191 and m/z 217 fragmentograms, respectively.

Inorganic geochemistry analyses

The whole crushed samples also were used for the analysis of major oxides and trace elements. X-ray fluorescence spectrometer (XRF) analysis was performed on the powdered sample using PANalytical Axios mAX 4kW sequential XRF spectrometer. The oxides of major elements and trace elements were identified from the non-destructive wavelength dispersive X-ray fluorescence spectrometer. In order to determine the low concentration of trace and rare earth elements, about 0.50 g of each sample was weighed in a Teflon beaker and dried at 105 °C overnight. The samples were moistened with a few millilitres of deionised water; 5 ml of nitric acid (HNO3) was slowly added and placed on a hotplate at 150 °C until it reaches near dryness and allowed to cool; 10 ml of hydrofluoric acid (HF) was slowly added followed by 4 ml of perchloric acid (HClO4). The samples were decomposed on a hotplate at approximately 200 °C until it reaches near dryness with a crystalline mush. After cooling, the samples were digested with 10 ml of 5 M HNO3 in a fume hood. The solutions were diluted with deionised water to 50 ml in a volumetric flask. All the digested samples were diluted up to 100 times with ultimate pure water (UPW). Standard solutions of the elements with an analyte concentration of 10 ppm were used for calibration with a minimum detection limit of less than 1 ppb. The trace and rare earth elements were determined using Agilent Technologies 7500 series inductively coupled plasma mass spectrometer (ICP-MS). The whole samples were subsequently used for the analysis of TS content using multi EA 2000 CS equipment.

Results and discussion

Molecular geochemistry characterisation

n-Alkanes and isoprenoids

The normal alkane and acyclic isoprenoid ratios of saturated hydrocarbon fractions of the extracts were determined based on distributions in m/z 85 of the GC-MS fragmentation (Fig. 3a). The chromatograms display full site of C13–C33 n-alkanes and isoprenoid hydrocarbons (Fig. 3a). The n-alkane distribution shows a unimodal distribution, with a predominance of short (n-C14–n-C20) to middle (n-C21–n-C25) chain n-alkanes with the presence of significant waxy alkanes (+n-C25) and thus gave low to moderate carbon preference index (CPI) values (Table 1). This n-alkane distribution supports high marine-derived organic matter contribution with terrigenous organic matter input (Ebukanson and Kinghorn 1986; Murray and Boreham 1992; Peters et al. 2005; Hakimi et al. 2012 2014).

Fig. 3
figure 3

Mass fragmentograms of a m/z 85, b m/z 191 and c m/z 217 of saturated hydrocarbons for representative two limestone extracts

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Table 1 n-alkane, isoprenoids and biomarker ratios of the limestone extracts from Banik Village in the Kurdistan Region calculated from m/z 85, m/z 191 and m/z 217 mass fragmentograms, respectively, illustrating source organic matter and depositional environment conditions
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Acyclic isoprenoids occur in significant amount in the terms of pristane and phytane isoprenoids (Fig. 3a). The analysed samples have pristane/phytane (Pr/Ph) ratios in the range of 0.60–1.84 (Table 1), indicating suboxic to anoxic conditions during deposition of the sediments. There are also significant differences in the amounts of n-alkanes compared to isoprenoids (Fig. 3a), thus giving relatively high pristane/n-C17 and low phytane/n-C18 ratios in the range of 0.42–1.50 and 0.28–0.58, respectively (Table 1). CPI index is also calculated to provide some insights into the source input of the organic matter (Peters et al. 2005). Late Jurassic-Early Cretaceous limestone sediments in the Kurdistan Region contain relatively low to moderate CPI values of 0.85–1.25 (Table 1).

Hopanoid and steroid biomarkers

The distributions of hopanoids, steroids and related compounds are commonly studied using GC-MS by monitoring the ions m/z 191 and m/z 217 mass chromatograms, respectively (Fig. 3b, c), and the identified peaks are listed in Appendix A.

The m/z 191 mass chromatograms of the saturated hydrocarbon fractions of all the analysed samples show that the hopanoids are dominated by the presence of C30 hopane, C29 norhopane, 17α (H)-trisnorhopane (Tm) and a considerable quantity of homohopanes (C31–C35) and tricyclic terpanes (Fig. 3b). However, the relative abundance of C29-norhopane is generally higher than that of C30-hopane in most of the studied samples (Fig. 3b), with C29/C30 17α (H)-hopane ratios in the range of 0.81–1.53 (Table 1). This is frequently associated with carbonate-rich rocks which agree with the lithofacies of the limestone samples. The enhanced norhopane C29 input may also be associated with land plant input (Rinna et al. 1996). Tm (C27 17a(H)-22,29,30-trisnorhopane) and Ts (C27 18a(H)-22,29,30-trisnorneohopane) are well known to be influenced by maturation, type of organic matter and lithology (e.g. Seifert and Moldowan 1978 1979; Moldowan et al. 1985). The investigated samples have varying Tm/Ts values in the range of 0.19–0.43 (Table 1), indicating that the samples contain a mixture of land- and aquatic-derived organic matter.

The homohopane distributions are dominated by the C31-homohopane and decrease with increasing carbon number (Fig. 3b). Homohopanes (C31–C35) are believed to be derived from bacteriohopanetetrol as well as from other hopanoids in bacteria (Ourisson et al. 1984). However, the C31-22R hopane/C30 hopane ratio is used to distinguish between different depositional environments (Peters et al. 2005). This ratio is generally higher than 0.25 for marine environments whereas lower than 0.25 for lacustrine settings (Peters et al. 2005). C31-22R hopane/C30 hopane ratios of the extract samples are in the range of 0.29–0.40, indicating that the studied limestone sediments were interpreted to be deposited principally in marine environment.

The concentration of tricyclic terpanes is relatively higher than that of tetracyclics in most of the samples analysed (represented by C24 tetracyclic/C26 tricyclic; Table 1). The samples also have relatively low C24 Te/C26 and C24/C23 tricyclic terpane values and relatively low C26/C25 tricyclic ratios (Table 1). Gammacerane also presents in small amounts in most of the samples (Fig. 3b), and the studied samples display variable gammacerane index (gammacerane/C30 hopane) in the range of 0.14–0.21 (Table 1). This is believed to have the existence of relatively high salinity stratified water column during deposition (Sinninghe Damsté et al. 1995; Ten Haven et al. 1988). This is evidenced by inorganic geochemical elements, which lead to the high Sr/Br ration and total sulfur contents (Tables 1 and 2).

Table 2 Major (wt%) and trace element (ppm) compositions of the Jurassic-Early Cretaceous limestone sediments from Banik Village in the Kurdistan Region, Northern Iraq
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The distribution of steroids is characterised by the m/z 217 ion chromatograms (Fig. 3c). Steroids are another group of important biomarkers that are derived from sterols found in higher plants and algae, but rare or absent in prokaryotic organisms (Seifert and Moldowan 1978 1979; Huang and Meinschein 1979; Volkman 1986). Steranes and diasteranes are present in high quantities in saturated hydrocarbon fractions of the analysed samples (Fig. 3c). The relative proportions of each of the regular steranes (C27, C28 and C29) can vary greatly from sample-to-sample, depending upon the type of organic matter input to the sediment. Relative abundances of C27, C28 and C29 regular steranes and the ratios of C29/C27 regular sterane, C27 + C28/C29 sterane, diasterane/sterane and sterane/hopane are calculated, and the results are given in Table 1. The analysed samples show a high proportion of C29 (38.8–48 %) and C27 (34.42–45.63 %) compared to C28 (12.3–18.5 %) steranes as shown in Table 1. The samples also have low diasterane/sterane and relatively high C29/C27 regular sterane ratios in the range of 0.43–0.82 and 0.88–1.41, respectively (Table 1). The low diasterane/sterane ratios further suggest carbonate-rich sediments.

Inorganic geochemistry characterisation

Total sulfur content

The total sulfur contents in sediments may provide insight into the depositional environment and microbial sulfate reduction (Berner and Raiswell 1983; Berner 1984; Mohialdeen, et al. 2013). High sulfur organic matter content is inferred to be derived from source deposited in highly reducing hypersaline marine (Huang and Pearson 1999), while the low sulfur concentrations are typical for a freshwater lacustrine environment (e.g. Berner and Raiswell 1983). Total sulfur is considered as a more strong measure of the degree of marine influence than the organic or pyritic sulfur in sediments. According to Sykes (2004), an adopted 0.5 % sulfur content was the highest concentration of sulfur in the entire non-marine sediments. As such, total sulfur values >0.5 % are taken to indicate some degree of marine influence, while values of 0.5–1.5 % and >1.5 % are classified as slightly and strongly marine influenced. The TS (wt%) content of the studied limestone samples ranges from 0.78 to 5.64 wt% (Table 1), thus showing that the samples were deposited under marine environment. Moreover, a high TS value is an indication of low oxygen bottom water conditions (Huang and Pearson 1999). These interpretations are supported further by the geochemistry of trace elements and biomarker environment indicators (Tables 1 and 2).

Major and trace elements

The origin and/or state of preservation and factors controlling the accumulation of organic matter and relation to paleoenvironmental conditions can be partly addressed by examining its bulk inorganic geochemical compositions (Moosavirada et al. 2011; Mohialdeen and Raza 2013; Shu et al. 2013; Jia et al. 2013). Inorganic geochemical results of studied limestone samples, including major oxides and trace elements along with the several widely used geochemical ratios, are tabulated in Table 3.

Major oxides (CaO, SiO2, Al2O3)

The major oxide elements CaO, SiO2, Al2O3 and Fe2O3 are the dominant constituents with an average of 67.73, 12.44, 8.18 and 4.57 wt%, respectively (Table 2). The carbonate input (CaO) showed highest concentrations in the studied samples (Table 3). This suggests that the samples were deposited in calcium-rich water (Stach et al. 1982). The high Ca concentration is explained by calcite carbonate minerals consistent with limestone sediments (Fig. 2). This also could be referred to the location of the depositional basin is more the depocenter of the basin.

SiO2 and Al2O3 showed slightly highest concentrations in the studied samples (Table 2), which is consistent with the occurrence of quartz and clay minerals. The Al/Si ratios are low (0.49–0.72), allowing to suggest that Si has another source besides clay minerals, as inferred by Fu et al. (2011). The slightly high concentrations of SiO2 and Al2O3 elements could be referred to the input of detrital matter (Ross and Bustin 2009). This is supported by relatively high titanium (Ti) element with an average 0.79 (Table 2) and very good correlations between Ti and SiO2 and Al2O3 elements (Fig. 4).

Fig. 4
figure 4

Relationship between major elements of titanium (Ti) and SiO2 and Al2O3 elements for investigated limestone sediments from Banik Village in the Kurdistan Region, Northern Iraq

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Iron

Fe2O3 is also an important component in the studied samples (avg. 4.57 wt%) and has known association with Fe sulfides (pyrite and marcasite). This is further suggest by the presence of pyrite within the Late Jurassic-Early Cretaceous limestone sediments (Mohialdeen 2008; Mohialdeen et al. 2013).

Phosphorus

Phosphorus contents also are detected in the Late Jurassic-Early Cretaceous limestone sediments (Table 2). The studied samples have low to moderate concentrations of phosphorus (P) in the range of 0.04–0.30, reflecting mixed organic matter input and leading to the formation of a reducing depositional environment favourable for preservation of organic matter (Shu et al. 2013). This is confirmed by organic matter input and conditions as indicated by biomarkers distributions (Table 1).

Vanadium and nickel

Vanadium and nickel are important trace elements to indicate the redox conditions during deposition as vanadium (V) is usually enriched in comparison with nickel (Ni) in anoxic marine environments (Barwise 1990; Peters and Moldowan 1993; Bechtel et al. 2001; Galarraga et al. 2008). According to Galarraga et al. (2008), a V/Ni ratio greater than 3 indicates that the sediments were deposited in a reducing environment, while V/Ni ratios ranging from 1.9 to 3 indicate deposition under suboxic conditions with precursor organic matter of mixed origin. The V/Ni ratios for the analysed samples are in the range of 1.74–5.76, thus showing suboxic to anoxic conditions during deposition. More so, high V/(V + Ni) ratios (0.63–0.85) and sulfur contents (1.01–2.70 wt%) in the studied samples denote that these samples contain carbonate-rich sediments that were deposited under suboxic to anoxic conditions (Fig. 5).

Fig. 5
figure 5

V/(V + Ni) versus sulfur content (TS; wt. %) of studied limestone samples from Banik Village in the Kurdistan Region, Northern Iraq

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Strontium and barium

Strontium and barium are two trace elements that are regarded as empirical indicators of paleosalinity (Liu et al. 1984; Deng and Qian 1993; Wang 1996). A high Sr/Ba ratio reflects high salinity, and a low Sr/Ba ratio indicates low salinity (Deng and Qian 1993). The studied samples have relatively high Sr/Ba ratio (average 1.67), indicating high saline water during deposition, thus have influenced by seawater. This is consistent with high total sulfur content values (>0.5). The Sr/Ba versus V/Ni ratios diagram (Fig. 6a) also reflects relatively high salinity stratification and relatively reducing conditions during deposition of the Late Jurassic-Early Cretaceous limestone sediments. This founding is also consistent with the presence of gammacerane biomarker (Fig. 6b), as gammacerane occurs in reducing and high salinity stratification during deposition (Sinninghe Damsté et al. 1995; Peters et al. 2005).

Fig. 6
figure 6

The diagrams of Sr/Ba ratio versus V/Ni ratio (a), and gammacerane index (b) for the limestone sediments of the Banik Village in the Kurdistan Region (adopted after Jia et al. 2013)

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Maturity of organic matter

In this study, the biomarker maturity parameters were used to assess the level of thermal maturity of organic matter within studied limestone sediments of the Kurdistan in the Northern Iraq (Mackenzie et al. 1980; Waples and Machihara 1991). The biomarker maturity ratios are C32 hopane 22S/(22S + 22R), moretane/hopane, C29 sterane ββ/(ββ + αα) and 20S/(20S + 20R) (Table 1). The ratios of 22S/(22S + 22R) for C32 hopanes are between 0.60 and 0.63 (Table 1), suggesting that they have already reached oil window (Seifert and Moldowan 1986). The ratios of C29 sterane ββ/(ββ + αα) and 20S/(20S + 20R), maturity ratios of the studied samples, are 0.47–0.58 and 0.39–0.46, respectively (Table 1), mostly either at or close to thermal equilibrium, consistent with their generation from early-mature to mature source rocks (Seifert and Moldowan 1978 1981) as shown in Fig. 7. This is supported by moretane/hopane ratios consistent with low relative abundance of C30 moretane (Fig. 3b). Moretane converts to C30 hopane with increasing thermal maturity (Seifert and Moldowan 1986), and thus, moretane decreases as thermal maturity increases. The ratio of moretane to their corresponding hopanes decreases with increasing thermal maturity, from about 0.8 in immature sediments to about 0.15–0.05 in mature source rocks and oils (Mackenzie et al. 1980; Seifert and Moldowan 1986). The limestones of the Iraq Kurdistan have moretane/hopane ratio in the range of 0.06 to 0.12, suggesting the samples have entered peak oil window maturity.

Fig. 7
figure 7

A range of thermal maturity based on two maturity-related biomarker parameters [C29 22S/(22S + 22R) and C29 ββ/(ββ + αα)] for the studied limestone extracts

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Origin of organic matter and paleodepositional conditions

In this study, organic geochemical (biomarker distributions) and inorganic data (major and trace elements) have been used to describe source input of organic matter and depositional environment conditions of limestones within the Late Jurassic-Early Cretaceous sequence in the Kurdistan Region, Northern Iraq.

The origin of organic matter was primarily examined through normal alkanes, hopanoids, steroids and related compounds (Waples and Machihara 1991; Hunt 1996; Peters et al. 2005). The n-alkane distribution patterns of saturated hydrocarbons can be used to define organic matter input from different producers (Brassell et al. 1978). The long-chain n-alkanes (>n-C23) are characteristic biomarkers for higher terrestrial plants (Eglinton and Hamilton 1967), whereas the short-chain n-alkanes (<n-C20) are predominantly found in algae and microorganisms (Peters et al. 2005). The n-alkane patterns of studied sediments are predominance by a short (n-C15–n-C20) to middle (n-C21–n-C25) chain n-alkanes with the presence of significant waxy alkanes (+n-C25) as shown in Fig. 3a. The n-alkane distributions are consistent with a dominant source of marine-derived organic matter, although receiving a minor terrigenous organic matter input (Ebukanson and Kinghorn 1986; Murray and Boreham 1992; Peters et al. 2005; Hakimi et al. 2012 2014). These interpretations are supported by low to moderate CPI values (0.85–1.25) (Table 1). The Pr/Ph ratios are in the range of 0.60–1.84 (Table 1), thus also further suggesting mixed organic matter with high contribution of aquatic organic matter deposited under suboxic to anoxic marine conditions (Didyk et al. 1978; Peters et al. 2005). The mixed organic matter type and conditions have also been interpreted using Pr/C17 and Ph/C18 ratios (Fig. 8). The influence of marine under reducing condition is also consistent with the high sulfur content (0.77–5.64 wt%) and the presence of gammacerane biomarker that were identified in m/z 191 mass fragmentograms (Fig. 3b).

Fig. 8
figure 8

Phytane to n-C18 alkane (Ph/n-C18) versus pristane to n-C17 alkane (Pr/n-C17) showing depositional conditions and type of organic matter of the analysed limestone samples from Banik Village in the Kurdistan Region, Northern Iraq

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The studied sample extracts have relatively high tricyclic terpanes compared to tetracyclic (Fig. 3b), consistent with a high contribution of an aquatic organic matter (Zumberge 1987). The presence of a relatively high C23 tricyclic terpanes (Fig. 3b) and low C26/C25 tricyclic terpane ratios (Table 1) is also indicative of a mixed marine organic matter (Aquino Neto et al. 1983; Marynowski et al. 2000). The high contributions of planktonic-bacterial organic matter is also demonstrated by the relative distribution of C27, C28 and C29 steranes in m/z 217 chromatograms of the samples (Fig. 9). In most studied samples, the C27 and C29 steranes predominate over the C28 steranes (Table 1), reflecting a high contribution of aquatic planktonic-bacterial organic matter with a minor terrigenous organic matter input (Peters and Moldowan 1993; Peters et al. 2005), as indicated by regular sterane ratio ternary diagram (Fig. 9; Huang and Meinschein 1979) and corroborated by C29/C27 sterane ratios (Fig. 10).

Fig. 9
figure 9

Ternary diagram of regular steranes (C27–C29) indicating the relationship between sterane compositions and organic matter input, showing that the analysed limestone extracts from Banik Village in the Kurdistan Region, Northern Iraq (modified after Huang and Meinschein 1979)

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Fig. 10
figure 10

Cross-plots of pristane/phytane ratios versus C29/C27 regular steranes of the studied limestone samples, showing that limestones contain high contribution of aquatic planktonic-bacterial organic matter with a minor terrigenous organic matter input

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The reducing marine depositional environmental conditions of the Late Jurassic-Early Cretaceous limestone sediments in the Kurdistan Region, Northern Iraq, have also been interpreted using geochemistry of major and trace elements. The high carbonate input (CaO) element with terrestrial detritus elements (e.g., SiO2, Al2O3 and TiO2) in significant amounts further confirms that the Late Jurassic-Early Cretaceous limestone sediments were deposited in calcium-rich seawater (Stach et al. 1982) with detrital materials input (Ross and Bustin 2009). The calcium-rich seawater has also been confirmed by high Sr trace elements (Table 2). Palaeo conditions during sedimentation of Late Jurassic-Early Cretaceous limestone sediments can also be evaluated from their trace element data such as V, Ni, Sr and Ba (Harris et al. 2004; Sageman and Lyons 2004; MacDonald et al. 2010; Fu et al. 2011). Some standard ratios such as V/Ni and Sr/Ba are most used as palaeo condition indicators in this study (Table 2). The high Sr/Ba and V/Ni ratios (avg. 1.67 and 3.21, respectively) reflect enhanced salinity stratification and anoxic conditions during deposition of the Late Jurassic-Early Cretaceous limestone sediments in the Kurdistan Region, Northern Iraq (Fig. 6a).

Conclusions

An integrated biomarker characterisation and inorganic geochemical elements of the Late Jurassic-Early Cretaceous limestone sediments in the Kurdistan Region, Northern Iraq, to infer origin, type and preservation of organic matter in relation to paleodepositional conditions have revealed the following:

  1. 1.

    The limestone sediments were deposited in a marine environment with calcium-rich seawater under suboxic to anoxic conditions as supported by biomarker environment characteristics and inorganic elements.

  2. 2.

    The saturated fractions of the limestone extracts are characterised by a predominance of short (n-C15–n-C20) to middle (n-C21–n-C25) chain n-alkanes with the presence of significant waxy alkanes (+n-C25), low to moderate CPI values (0.88–1.25), relatively low Pr/Ph ratios (0.60–1.84) and high concentrations of regular sterane C27, as well as the presence of tricyclic terpanes, which is consistent with reducing marine environment which de-signifies a dominant contribution of aquatic organic matter. A small amount of terrigenous organic matter input is present based on the n-alkane distributions and significant concentrations of regular sterane C29.

  3. 3.

    The dominant major elements identified in the limestones are CaO (avg. 67.73 wt%) followed by SiO2 (avg. 12.44 wt%), Al 2 O 3 (avg. 8.18 wt%) and Fe2O3 (avg. 4.57 wt%). The assemblages and modes of occurrence supported the calcium-rich seawater origin. Minor detrital material input is also evidenced based on the significant amounts of SiO2 and Al2O3 elements and relatively high T) element (avg. 0.79 wt%).

  4. 4.

    Based on the assessment of Sr, Ba, V and Ni trace elements and their ratios, a stratified water column with salinity and low oxygen bottom water conditions within the Late Jurassic-Early Cretaceous limestone sediments are evidenced and again, indicative of a relatively reducing marine condition.