Research paper
Organic geochemistry and basin modeling of Late Cretaceous Harshiyat Formation in the onshore and offshore basins in Yemen: Implications for effective source rock potential and hydrocarbon generation

https://doi.org/10.1016/j.marpetgeo.2020.104701Get rights and content

Highlights

  • Late Cretaceous Harshiyat Formation source rock in the onshore and offshore basins of Yemen was investigated.

  • Generation and expulsion of oil started from Late Oligocene and ended in Late Miocene.

  • Gas window evolution started from Late Miocene and continued to present day.

  • Oil exploration targets should concentrate on the Harshiyat Formation along the deeper parts of the basins.

Abstract

The purpose of this study is to present the source rock characteristics of Late Cretaceous Harshiyat Formation source rock in the onshore and offshore basins of Yemen (i.e. Mukalla-Sayhut, Sayun–Masilah , and Jiza-Qamar). Source rock geochemical characterizations were completed and integrated to perform 1-D basin modeling to back-predict source rock thermal maturation, oil generation and expulsion.

The geochemical findings revealed that mudstone intervals within the Harshiyat Formation comprised favorable source rocks, with total organic carbon (TOC) content between 0.50 and 35.10 wt %. The organic matter intervals in the Harshiyat mudstones are mainly Types II/III and III kerogens, with certain amount of Type II and I kerogens based on Rock-Eval HI values, ranging from 40 to 923 mg HC/g TOC. Mixed kerogen types were consistent with vitrinite and sapropelic organic matters, as observed by visual kerogen examination and the distribution of biomarker fingerprints. The existence of mixed types I, II, and III organic matter suggests that the Harshiyat mudstones could contribute to oil and gas at sufficient thermal maturity levels. The biomarker data of the organic matter in the Harshiyat mudstones show that the source rock contains a mix marine phytoplanktonic-bacteria/terrestrial land plant enthronement and were deposited in suboxic-anoxic conditions.

Results of chemical and optical maturity indicators led to the conclusion of varying maturity stages from immature to late oil window levels for the mudstones of the Harshiyat Formation. Thermal maturation distributions inferred from the analyzed samples indicate that mudstone intervals in the offshore Mukalla-Sayhut Basin are extremely mature, compatible with the late-mature oil window and may have generated significant amounts of oil. This interpretation was supported by strong geochemical similarity between the organic matter in the mudstones and those of the oils found in the offshore Mukalla-Sayhut Basin and demonstrated by the concentrations of n-alkanes and isoprenoids as well as carbon isotopes.

In the offshore Ras-Ghashwah-1X well, burial and thermal models were constructed and used to address the effect of tectonic events in triggering oil generation and expulsion from mature Harshiyat source rock in the offshore area. Kinetic database models for Type II and III kerogen mixtures indicate that the initial conversion of kerogen to oil occurred since the Late Oligocene with transformation ratios (TR) that span between 10% and 50%. Furthermore, the oil was expelled from organic matter intervals in the Harshiyat source rock starting from Late Oligocene and continuing until Late Miocene, with TRs of 50%–72%. From that period up to the present, the retained oil was cracked into wet gas in the gas window with an Easy %Ro greater than 1.30, and with peak TR ratios greater than 72%.

Introduction

Petroleum exploration in Yemen has been intermittently active since 1961 when exploration started along the southern coastal region of the Red Sea (YOGC, 2008; SPT, 1994). Onshore exploration endeavors preceded this, particularly in the Sabatayn Basin in western Yemen (Fig. 1A) with the discovery of much oil and gas fields in the Marib-sector of the basin in 1982. Among these fields are the Alif Field, which produced 8000 BOPD and estimated hydrocarbon resources of about 3 billion barrels of oil and condensate, and 11 trillion ft3 of gas (YOGC, 2008; PEPA, 2012). The exploration activity for conventional petroleum resources was extended to other onshore and offshore basins in the southern and eastern parts of the Republic of Yemen (Fig. 1A).

The stratigraphic successions of the onshore and offshore basins preserve a thick stratigraphic succession of economic importance and contain several petroleum-bearing reservoir and source rock units of Mesozoic and Cenozoic age. Source rocks that occur in the stratigraphic columns of the basins contain coal, carbonaceous shale, and organic-rich shales (Brannin et al., 1999; Csato et al., 2001; Alaug et al., 2011; Alaug et al., 2013; Hakimi and Abdullah, 2013a; Hakimi et al., 2010, Hakimi et al., 2014, Hakimi et al., 2015, Hakimi et al., 2017, Hakimi et al., 2018a, Hakimi et al., 2020a Hakimi and Ahmed, 2016; Hatem et al., 2016; Al Areeq et al., 2019). The two onshore Sabatayn and Sayun–Masilah basins preserve the main petroleum-containing stratigraphic sequences and contain proven commercial quantities of hydrocarbons with high production from the largest oil fields in Yemen (e.g. King et al., 2003; Hakimi et al., 2011; Hakimi and Abdullah, 2013b; Al Areeq and Maky, 2015).

In the last two decades, extensive geochemical, organic petrological, and basin modeling endeavors considered the characteristics of particular source rocks in the Late Jurassic Madbi and Sabatayn units (Alaug et al., 2011; Hakimi and Abdullah, 2013a, 2015; Hakimi et al., 2010, 2014, 2018b, 2020a; Hatem et al., 2016; Al-Areeq et al., 2018, 2019). It has been widely documented that the organic-rich shale intervals within the Late Jurassic Madbi Formation form the main source rocks for conventional petroleum accumulations in the Sabatayn and Sayun–Masilah basins (Alaug et al., 2011; Hakimi et al., 2010, 2014; Al Areeq et al., 2019). It is also documented that the organic matter of the source rocks of the Madbi Formation is of a marine origin (Hakimi et al., 2014, 2020a; Hatem et al., 2016). The analysis of the organic-rich shale intervals of the Madbi Formation that were collected from the flanks of the structural highs of the Sabatayn and Sayun–Masilah basins indicated that they became fully mature, readily yielded oil, and then cracked oil to gas in the kitchen areas of the basins (Hakimi et al., 2010, 2014; Hakimi and Abdullah, 2015; Al-Areeq et al., 2018, Al-Areeq et al., 2019).

Recently, studies on the geochemical characteristics of new potential petroleum resources in different onshore and offshore basins in Yemen have been performed on the Cretaceous and younger units. These current studies intend to increase our knowledge and documentation on potential hydrocarbon reserves and development of petroleum exploration in the basins in Yemen (e.g. Hakimi and Ahmed, 2016; Hakimi et al., 2016; Al-Matary et al., 2018; Hakimi et al., 2019; Hakimi et al., 2020b,c). These Cretaceous and Tertiary rock sequences contain intervals rich in organic matter and have highly promising source rocks that could generate and expel petroleum if sufficient petroleum-generating parameters were complete. Unfortunately, these sediments have not been buried to a sufficient depth to reach high maturity for generation and expulsion of commercial petroleum (Hakimi and Ahmed, 2016; Hakimi et al., 2016; Al-Matary et al., 2018; Hakimi et al., 2019; Hakimi et al., 2020b,c).

In this study, we geochemically analyze and document undiscovered or overlooked potential petroleum resources in Yemen's basins. This study focused on the source potential of stratigraphic units within the sandstone-dominated, Late Cretaceous Harshiyat Formation that occurs in the onshore and offshore basins in Yemen (Fig. 1B). This study aimed mainly to: (1) provide information on source rock potential by determining the type and richness of organic matter, thermal maturity, and petroleum generation potential of mudstone units within the Harshiyat Formation; (2) assess organic matter inputs into their depositional environment; and (3) infer the genetic relationships among oils and those of the analyzed Harshiyat mudstones by using the biomarker fingerprints and stable carbon isotope (δ13C) composition of aliphatic and aromatic fractions in this study and the available published work on the oils (e.g. Hakimi et al., 2018c). We also conducted a 1-D basin modeling at the site of the deep source rock in the offshore well (Ras-Ghashwah-1X). Expectedly, the modeling sheds light on the source rock maturation history and the influence of tectonic events in triggering oil generation in the offshore region.

Section snippets

Geological setting

The separation of the Indian-Madagascar continent from the eastern Afro–Arabian part of Gondwanaland resulted in roughly northwest-southeast-oriented fracture zones that evolved into extensional regime and opening of rift basins. Mesozoic to middle Paleogene basins extended from northern Somalia into eastern Arabia (Beydoun, 1970; Beydoun et al., 1996; Salad Hersi and Leckie, 2008), whereas late Paleogene (mainly post-Eocene) basins were related to the opening of the Red Sea-Gulf of Aden

Methods and dataset

A total of 100 ditch cutting samples retrieved from the mudstone intervals of the Late Cretaceous Harshiyat Formation were collected from nine exploration wells (i.e. Kharir-02, Kharir-03, Sunah-01, Al Qarn-01, Ghayl Bin Yumain-01, Al-Armah-01, Al-Furt-01, Ras-Ghashwah-X1, and Sarar-X1) in onshore Sayun–Masilah and Jiza-Qamar basins and offshore Mukalla-Sayhut Basin (Fig. 1B and Table 1). These nine wells were drilled using water-based mud, and the analyzed mudstones were picked out from the

Organic facies petrology

Organic petrological analysis is mainly used to identify the organic facies of the mudstones of the Harshiyat Formation, and their petroleum typing and maturation. The identification of kerogen assemblages in the analyzed mudstone samples and their vitrinite reflectance (%VR) values are tabulated in Table 2.

The petrographic analysis of the mudstone samples indicates that the kerogen assemblages that occur in the samples are dominated by vitrinite, which volumetrically lies in 20%–90%. This is

Origin of organic matter and depositional environment

The origin of organic matter in the analyzed Harshiyat mudstones and their depositional conditions were primary assessed from biomarker distribution components of the saturated hydrocarbon fraction (e.g. Waples and Machihara, 1991; Peters and Moldowan, 1993; Peters et al., 2005).

The normal alkanes show unimodal distributions, with high abundance of short-and medium-chain n-alkanes (Fig. 9A). This suggests that the analyzed samples have a mixture of organic matter with a high abundance of

Conclusions

Organic geochemical and optical analyses and basin modeling technique were conducted on the mudstone intervals of Late Cretaceous Harshiyat Formation gathered from nine exploration wells drilled in the Mukalla-Sayhut, Sayun–Masilah, and Jiza-Qamar basins in Yemen. The following points were concluded:

  • 1

    The Harshiyat mudstones are classified as having fair to excellent potential source rocks as indicated by the TOC content between 0.50 and 35.10 wt percent. These mudstone sediments contain mixed

Authorship contribution statement

Mohammed Hail Hakimi: Recommend the study idea, Drawing all the manuscript illustrations and graphs, Constructing all the basin models and their dissuasion, Writing the first draft of the manuscript, Final writing of the research and ensure its integrity from mistakes and grammar errors and good order in its final manuscript. Adeeb Ahmed: Recommend the study idea, Drawing all the manuscript illustrations and graphs, Constructing all the basin models and their dissuasion, Writing the first draft

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The senior author is thankful for supplying the geochemical data in this work to the Total Oil Exploration and Production Company and Petroleum Exploration and Production Authority (PEPA), Republic of Yemen. Special acknowledgments are offered to Schlumberger for providing the use of the free 1-D PetroMod software. The authors extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this work through Research Group No. (RG-1438-059). Drs.

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