# Introduction emen economies are reliant mostly on oil production. The annual petroleum consumption was over 168000 barrel per day in Yemen of 2011 census (Yemeni petroleum exploration & production Authority, (PEPA). The petroleum exploration and production activities have been affected by security issues since 2011, remarkable drop have affect the country economy as well. Worse still, the traditionally large Yemeni oilfields, including Alif, Kharir and Halewah fields are facing a crisis of production reduction. Therefore, resource reassessment must carry out in parts of sedimentary basins previously little explored especially in northwestern part of the petroliferous Sab'atayn basin Fig. 1. The Sab'atayn basin, which conserved Mesozoic succession in its stratigraphy, favored petroleum accumulation because it contains the whole petroleum system element (Source, Reservoirs and Seal rocks). The upper Lam member is the first target of source rock assessment and hydrocarbon exploration because of organic matter richness and greater proli fic oil prone source rock across Yemen (Brannin et al., 1999; Albaroot et al., 2016). The Lower Meem member made up of argillaceous limestone (Alaug et al., 2011; Al-Areeq, 2011; Al-Azazi, 2010 and Al-Areeq, 2004), consider the second target of source rock assessment. In the past decades, several wells have been drilled in northwestern (NW) part of Sab'atayn basin but unfortunately the results became frustrated. Due to the necessity to increases oil potential we try to re-evaluate this part of the basin by using the available geochemical data from the source rocks. Therefore, it is necessary to evaluate systematically the characteristic of the source rocks and their maturity evaluation within this part of the basin. This evaluation can improve our understanding of Lam and Meem source unit evolution and maturation. The characteristics of source rock evaluation include the kerogen type, organic matter abundance and source rock maturity. The source rocks thermal maturity investigation primarily includes vitrinite reflectance (% Ro) and temperature maximum (Tmax) from the Rock-Eval pyrolysis. The quantity of organic matter is commonly assessed by a measuring total organic carbon (TOC) contained in the rocks. Quality is measured by determining the types of kerogen contained in the organic matter. Thermal maturity is most often estimated by using vitrinite reflectance measurements in addition to data from pyrolysis analyses. However, drilling wells and samples are short in the NW part of the basin, because this area has not been subjected to extensive conventional oil and gas targets. Therefore, it is impossible to do any geochemical analysis and difficult to study using conventional experimental test methods due to core samples chips scarcity, only data of geochemical analysis can be obtained from (SPT, 1994) reports. Challenges and breakthroughs in recent research in hydrocarbon generation, expulsion, migration and accumulation led to more understanding the whole process of hydrocarbon. Therefore, source rock investigation is of increasing importance because it reduces risk potential and gives a quick insight of concerned area. Along with the development of petroleum geology theory and the wide application of computing technology, quantitative research on the thermal maturity evolution of source rocks in the geological period is of great significance. # II. # Geological Setting Yemen, situated at the southern end of the Arabian Peninsula, both are geographically and geologically has the same geological signature between the Arabian and African plate. (Redfern and Jones, 1995). During the Late Jurassic commencing in the Kimmeridgian, syn-rift sediments of the Madbi Formation were deposited . The Madbi Formation is composed of porous limestone to argillaceous lime mudstone. This Formation is divided into two members, the lower member (Meem Member) consists of source rock-quality shales, and sandy turbidites in the border of the basin and may form the reservoir rocks in some Volume XXII Issue IV Version I oilfields of the northwestern Sab'atayn Basin. The Upper Lam Member is mostly composed of laminated organic rich shales and considered to be the most prolific oilprone source rock in the basin (Brannan et al., 1999 andCsato et al., 2001). During Tithonian time, late stages of the syn-rift phase, ocean circulation in the Sab'atayn Basin became restricted, and an evaporitic succession (Safir Member) with an estimated original thickness of about 731 m was deposited (Albaroot, 2017). Massive halite occurs in the basin center, whereas anhydrite and clastic rocks rare along the basin margins (Seaborne, 1996), or totally absent. Interbedded thin shales within Safir member are rich in organic matter (Brannan et al., 1999). The Sab'atayn Formation is divided into four members named as Safir, Alif, Seen and Yah Member. Yah Member is dominated by fluvio-deltaic sandstone, mudstone and evaporate, followed by Seen Member, which is the second clastic sequence. Alif Member is composed of sandstone with shale, which form main reservoir in Sab'atayn Basin. Safir Member consists predominantly of halite with subordinate anhydrite divisible into several bodies separated by interbedded organic-rich shale and sandstone with minor argillaceous, dolomite and limestone. The interbedded organic rich shales within the Safir Member are considered to be the prolific oil-prone source rock in the Marib-Shabwa Basin within Sab'atayn Formation. The Safir Member constitute an excellent seal to the underling Alif Member reservoir and contain within them some potential good local reservoir seal pairs in the intra evaporate clastics and the evaporates . In the Northwestern part of the Sab'atayn basin during Tithonian time, deposition of late stages of the syn-rift phase clastic and evaporates sedimentations (Sab'atayn Formation) didn't extended and progressively thinned out for causes not well understood. # III. Methology provide information on the quantity, quality and maturity of organic matter contained within the Lam and Meem rock units (Table 1). A total of 148 rock samples were collected from shales of the Lam and Meem Members in the studied wells. Initially, the studied shale samples were cleaned of contaminants from drilling mud additives by washing the samples with water several times until no mud was visible on their surface. Parameters measured include Total organic carbon (TOC), free hydrocarbons (S1) in the rock, remaining hydrocarbon generative potential, mgHC/g rock (S2), and temperature of maximum pyrolysis yield (Tmax). Hydrogen (HI), production yield (PY), and production (PI) indexes were mathematically calculated (Table 1). The temperature at which the maximum generation of the products of pyrolysis occurs was used to calculate IV. # Results & Discussions The capability of any prospective reservoir depends on an effective source rock. Petroleum geochemistry is proving its value in helping petroleum geologists to evaluate source rocks and quantify the elements and processes that control the generation of oil and gas. Geochemistry is also an important tool for reducing uncertainty inherent in exploration and production of frontier basins. This section will explore basic geochemical methods used to evaluate new prospects. # a) Quality and quantity of organic matter The impact of quality and quantity of the organic matter (TOC) in the sediments are very important for hydrocarbon generation. The quality term of organic matter is refer to whether the source rock organic matter is oil prone or gas prone, since different types of organic matter have different hydrocarbon generating potential or quality. However, the amount of organic matter in source rocks is the results of a wide variety of environmental influences. Tissot and Welte (1984), Peters and Cassa (1994) and Peters (1986) presented a scale for the assessment of source rocks potentiality, based on the TOC weight % and Rock-Eval Source rock evaluation within the study area depends on the determination of organic matter content, which is usually expressed as total organic carbon (TOC). The hydrocarbon potentiality depends on the type and quantity of organic matter (kerogen) preserved in the petroleum source rock, thermal maturity and finally the generation potential of kerogen. The geochemical data such as total organic carbon (TOC), Rock-eval pyrolysis data, and vitrinite reflectance are presented and discussed for the proposed Upper Jurassic rock units in Northwestern part of Sab'atayn Basin (Dahamr Ali-01, Himyar-01, Kamaran-01 and Saba-01 wells). TOC determination and Rock-Eval pyrolysis analysis were performed on 100 mg crushed whole rock samples, heated to 600°C in a helium atmosphere, using a Rock-Eval II unit with a total organic carbon module. The Rock-Eval pyrolysis data pyrolysis data, such as S 1 and S 2 . The obtained data in (Table 1) show that the total organic carbon content (TOC) values for the Meem source rocks are between 0.2 and 1.68 wt% indicating fair to good source rocks. While the values for the Lam source rocks are between 0.2 and 2.93 wt% indicating fair to excellent source rocks only two samples have values more than 3 wt% in Kamaran-01 well. These conclusions are confirmed by the plots of total organic carbon (TOC wt%) versus remaining hydrocarbon (S2 mgHC/g rock) Fig. 2A. The total organic carbon is mostly very poor in studied wells. The Rock-Eval pyrolysis data in (Table 1) reveal that most of the samples consist of reworked organic matter with no interesting source rocks potential. On the other hand, the plot of Tmax versus production index (PI) Fig. 2B provides an indication of source rock maturity and hydrocarbon genesis. Thermal maturity is influenced by source rock organic matter type and the presence of excess free hydrocarbon together with the other factors like mineral matter, content, depth of burial and age (Tissot and Welte, 1984). The degree of thermal evolution of the sedimentary organic matter was deduced from Tmax (°C) Production Index (PI) and Vitrinite Reflectance (% Ro). The increase of maturity level of organic matter corresponds to an increase in Tmax. This phenomenon is related to the nature of chemical reactions that occur through thermal cracking. The weaker bonds breakup in the early stages while, the stronger bonds survive until higher temperatures in the late stages (Whelan and Thompson, 1993). Combining and finding relations between the essential Rock-Eval parameter, Tmax, and calculated Rock-Eval parameter, PI, is a valuable method for indicating the maturity of organic matter. The following relations between Tmax and PI are observed: In well Dahamr Ali-01, most of the samples of Meem source rocks especially in the lower part have Tmax more than 445 °C and PI of 0.34 -0.73. This indicate that the lower part in mature stage, while the upper part are in early mature and immature stages. Most of the samples are non-indegenous hydrocarbon except for few samples which fall within the hydrocarbon generation zone. Most of the samples in Himyar-01, Kamaran-01 and Saba-01 wells have Tmax less than 445 °C, accordingly ranging from immature to early mature stage. Some samples have elevated Tmax more than 445 °C making them peak mature. Samples from aforesaid wells except four samples from Kamaran-01 well are in main stages of hydrocarbon generation. The reset samples are non-indigenous hydrocarbons (Fig. 3). Most of the samples from the Lam source rocks in Dahamr Ali-01, Himyar-01, Kamaran-01 and Saba-01 wells are have Tmax less than 435 °C, accordingly plotted in immature zone. # b) Generating potentialities The generating potential of source rocks is used to evaluate their capacity for hydrocarbon generation and can be determined by using the results of pyrolysis analysis. Tissot and Welte, (1984) proposed a genetic potential (GP = S1 + S2) for the classification of source rocks. According to their classification scheme, rocks having GP of less than 2 mg HC/g rock correspond to gas-prone rocks or non-generative ones, rocks with GP between 2 and 6 mg HC/g rock are moderate source rocks, and those with GP greater than 6 mg HC/g rock are good source rocks. Based on the above criteria, the Meem source rocks with a GP of less than 2 are nongenerative rocks. Furthermore those source rocks with exceptionally high GP values in order of more than 10 mg HC/g rock may provide either an excellent source rock in Dahamr Ali-01 well, if the burial depth is sufficient to build temperature and pressure. On the other hand Lam source rock is classified as moderate source rocks. Non-generative potential has been reported for Lam source rock in Himyar-01 well where the GP is less than 1 mg HC/g rock Fig. 3. It is worthy to mention that both of the source rocks are located in shallow depth in the study area even more exposed on the surface for some wells location. # c) Genetic type of organic matter The initial genetic type of organic matter of a particular source rock is essential for the prediction of oil and gas potential. Waples, (1985) used the hydrogen index values (HI) to differentiate between the types of organic matter. Hydrogen indices <150 mg/g indicate a potential source for generating gas (mainly type III kerogen). Hydrogen indices between 150 and 300 mg/g contain more type III kerogen than type II and therefore are capable of generating mixed gas and oil but mainly gas. Kerogen with hydrogen indices >300 mg/g contains a substantial amount of type II macerals and thus are considered to have good source potential for generating oil and minor gas. Kerogen with hydrogen indices >600 mg/g usually consists of nearly type I or type II kerogen; they have excellent potential to generate oil. Kerogen type for Lam and Meem source units can be deduced by the cross-plots of pyrolysis parameters, such as HI vs Tmax (modified ven Krevelen diagram, Fig. 4 and TOC vs S2 (Fig. 2A) which are probably resulted from deposition of more terrigenous type III organic matters sourced from land. Type III kerogen is Volume XXII Issue IV Version I 4 ( ) composed of terrestrial organic material that is lacking in fatty or waxy components. Cellulose and lignin are major contributors. Type III kerogen have much lower hydrocarbon-generative capacities than do Type II kerogen and, unless they have small inclusions of Type II material, are normally considered to generate mainly gas. Majority of study area is dominated by type III kerogen, which is attributed to terrestrial environment where land derived organic matter is prevailed. This type of kerogen is characterized by small amount of Hydrogen is present; However this type of kerogen can generate gas only. # d) Thermal Maturation Thermal maturity is the extent of heat-driven reactions that alter the composition of organic matter. The concentration and distribution of hydrocarbons contained in a particular source depend on both the type of the organic matter and its degree of thermal alteration (Longford et al, 1990). In the present paper, the thermal maturity level of the source rocks of Meem and Lam members has been determined by the study of the geochemical parameters as Rock-Eval temperature pyrolysis ''Tmax'', Hydrogen index ''HI' Fig. 4. Combining and finding relations between the essential Rock-Eval parameter, Tmax, and calculated Rock-Eval parameter, HI, is a valuable method for indicating the thermal maturity of organic matter. Based on pyrolysis data kerogen classification diagrams were constructed using the HI versus Tmax plot as carried out by previous workers (Espitalie et al, 1985) which is used to determine the kerogen type and maturity Fig. 4. The results show that the analysed Meem source rocks are generally plotted in the mature zone of type III kerogen. Some samples in Dahamr Ali-01 well are upgraded to marginally mature zone. In addition Kamaran-01 well ranges from mature to post mature zone. The wide variation in maturity level of Meem source rocks attributed to overburden rocks and depth. Results of Lam source rocks samples show that the source rocks are still immature. Marginally mature in Dahamr Ali-01 and Saba-01 wells. These results have led to classify the Meem member as fully mature source rocks, while the Lam member is immature source rocks in the study area, because the structural setting shows the deepening of Meem member and shallowing of Lam member. # V. Conclusion Upper Jurassic source rocks in the NW Sab'atayn Basin central Yemen have been investigated. The main conclusions of the study are, Upper Jurassic source rocks consider the main source rocks in the study area. Deposition of the Meem and Lam source rocks succession did not result in a renewal of generation processes. As evident from kerogen type present in studied wells we can clearly argued this kerogen derived from land derived organic matter. The Rock-Eval pyrolysis data is reveal that most of the samples consist of reworked organic matter with no interesting source rocks potential. Organic rich source rock with poor to good potential to generate oil and gas is present in the Upper Jurassic Meem and Lam Members. Good to fair source rocks of Meem and Lam Members is located in study area. Results of TOC for the studied wells show that the quantity of source rocks are fair to good, some samples are graded to excellent. Most of the studied samples of Meem and Lam source rocks have Tmax less than 440 °C, which place them in immature to marginally mature. Majority of samples in main stages of hydrocarbon generation Based on generative potential of Meem source rocks, it shows non-generative rocks. Kerogen type for Lam and Meem source units is dominated by type III organic matters sourced from land. Volume XXII Issue IV Version I ![It is located close to the Tertiary triple junction between the Red Sea, Gulf of Aden and Afar plume. The NW-trending Sab'atayn Basin (also called Marib-Shabwa-Hajar Basin, Marib-Shabwa Basin) is a Mesozoic rift basin in Yemen that follows a deep-seated Proterozoic structural trend (Redfern and Jones, 1995) and oriented NW-SE (Bott et al., 1992; Brannan et al., 1999). Basin formation was a consequence of several extensional phases related to the separation of India from Africa-Arabia (Redfern and Jones, 1995; Ziegler, 2001). The stratigraphic framework of the basin fill according to Beydoun et al. (1998) it varies from marginal to central. The development of Sab'atayn Basin can be subdivided into three tectonostratigraphic megasequences: (a) a pre-rifting phase (Permian-Oxfordian/Kimmerdgian), (b) a syn-rifting phase (Kimmerdgian-Tithonian) and (c) a post-rifting phase (Early Cretaceous), Permian pre-rift phase not well proved. In the NW part of the basin, syn-rifting phase parts (Kimmeridgian-Tithonian) and post-rifting phase (Early Cretaceous) not encountered at all due to uplift and erosion. Pre-rift deposits are represented by non-marine to shallow-marine clastic rocks (Kuhlan Formation; (Beydoun et al., 1998)) overlain by shallowmarine carbonates (Shuqra Formation](image-2.png "") ![Upper Jurassic Source Rock Evaluation and Thermal Maturity Evolution of the NW Sab'atayn Basin, Yemen](image-3.png "B") 1![Figure 1: Location map of the studied wells in the NW Sab'atayn Basin, central Yemen.](image-4.png "Figure 1 :") ![the following: OI, oxygen index [OI = (S3/TOC) x 100)] HI, hydrogen index [HI = (S2/TOC) x 100]. Plot of HI versus OI can be used to deduce the type of organic matter present in the source rock. PI, production index [PI = S1 / S1+S2) Hydrogen richness in the kerogen = S2/S3 Genetic potential of the source rock = S1+S2 Details on the Rock-Eval method and parameters as well as a summary of interpretive guidelines for Rock-Eval data are available in (Espitalie et al. (1977), Peters (1986), Peters and Cassa (1994), and Snowdon et al. (1998).](image-5.png "") 1Wells Name Members Depth (m)TOC "wt%"S1S2 S1+S2 Tmax HIPI BUpper Jurassic Source Rock Evaluation and Thermal Maturity Evolution of the NW Sab'atayn Basin, Yemen ## Acknowledgement The authors thank the Ministry of Oil and Minerals and Petroleum Exploration and Production Authority (PEPA), Yemen for supplying the raw and approval and permission to use the material for studied area. The authors also would like to sincerely thank the editors and all of the anonymous reviewers for their careful and useful comments that improved the revised manuscript. * Source rock evaluation, maturation, and reservoir characterization SAlaug Leythäeuser AFBruns Ahmed Iran. J. Earth Sci 3 2011 Source and reservoir rocks of the Block 18 oilfields * Petroleum Formation and Occurrence PTissot DHWelte Springer New York second ed. 1984 * The petroleum potential of onshore Tasmania: a review. 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