ارزیابی ژئوشیمیایی سنگ منشأ نخستین گام در بررسی سیستمهای هیدروکربنی طی برنامههای اکتشافی است. سازند پابده با سن پالئوسن تا الیگوسن و سنگشناسی مارن، آهک و شیل یکی از سازندهای محتمل برای زایش هیدروکربن در سواحل شمالغربی خلیج فارس و جنوب فروافتادگی دزفول است. در پژوهش حاضر، توان هیدروکربنزایی، کمیت و کیفیت سنگ منشأ، نوع کروژن، بلوغ حرارتی و گسترش رخسارۀ آلی سازند پابده بررسی شدند؛ به این منظور، تعداد 47 نمونه از میدانهای واقع در نواحی موردمطالعه با دستگاه راک- اول 6 تجزیهوتحلیل شدند. نتایج نشان دادند کروژنها از نوع II، III و مخلوطی از کروژن III و II هستند، ولی اکثراً نابالغند و در مرحلۀ دیاژنز قرار گرفتهاند. پتانسیل هیدروکربنزایی 90 درصد نمونهها در بخش متوسط تا عالی قرار دارد. رخسارههای آلی این سازند در محدودۀ B، BC، C و CD قرار دارند. این تنوع رخسارههای آلی از تغییرات جانبی رخسارهها و تغییرات نسبی سطح آب دریا طی زمان رسوبگذاری این سازند ناشی میشود. سازند پابده در منطقۀ مطالعهشده به سه زون ژئوشیمیایی تقسیم میشود: زونهای بالایی و پایینی در زمان پسروی آب دریا با مقادیر TOC و HI کم و کروژن نوع III تهنشست یافتهاند؛ درحالیکه زون میانی در زمان پیشروی آب دریا با مقادیر TOC و HI زیاد و کروژن نوع II نهشته شده است. سازند پابده در منطقۀ مطالعهشده سنگ منشأیی با پتانسیل هیدروکربنزایی خوب ولی نابالغ شناخته میشود و زون میانی این سازند منبع هیدروکربنی غیرمتعارف در نظر گرفته میشود.
عنوان مقاله [English]
Geochemical Evaluation and organic facies distribution of the Pabdeh Formation in northwestern coastal Part of the Persian Gulf and southern Dezful Embayment by Rack-Eval Analysis
Geochemical evaluation of source rock is the first step in assessment of hydrocarbon systems for their exploration. The Pabdeh Formation with the age of Paleocene to Oligocene and lithology of marl, limestone and shale is one of the potential source rocks in the southern part of the Dezful Embayment and northwestern coastal part of the Persian Gulf. In this research, the hydrocarbon generation potential, quality and quantity of source rock, kerogen type, thermal maturation and organic facies distribution are studied. For this purpose, 47 samples from different oil fields in the studied area were analyzed by Rock-Eval 6 pyrolysis system. The kerogens are mostly type II, III and II/III, however the samples are mainly immature and are within diagenesis stage. The 90% of samples have fair to excellent hydrocarbon generation potential. The organic facies of the formation are in the rage of B, BC, C and CD. The organic facies variations are due to lateral facies changes and sea-level fluctuations during deposition of this formation. The Pabdeh Formation can be divided into 3 geochemical zones. The upper and lower zones were deposited during sea level regression with low quantity of TOC and HI as well as kerogen type III. However, the middle zone that deposited during the sea level transgression has high quantity of TOC and HI with kerogen type II. The Pabdeh Formation in the studied area can be characterized as high hydrocarbon generation potential but immature source rock and the middle zone of this formation may represent an unconventional hydrocarbon reserve.
Keywords: Hydrocarbon generation potential, Pabdeh Formation, Rock-Eval, Organic facies, Kerogen.
Geochemical assessment of source rock is an interdisciplinary approach which is utilized for increasing success rate of hydrocarbon exploration and efficiency of development plans. This technique can help in determining source rock distribution, source rock maturity, amount of organic matter, hydrocarbon potential generation, kerogen type, amount of production and migration pathways. On the other hand, the regional evaluation of the source rock in the sedimentary basin is necessary because different part of the basin may have different hydrocarbon potential. For this purpose, in this research, the geochemical conditions and organic facies distribution of the Paleocene to Oligocene Pabdeh Formation in northwestern coastal part of the Persian Gulf and southern Dezful Embayment have been investigated.
Material and Methods
In this research 47 samples from seven wells in the southern part of the Dezful Embayment and northwestern coastal part of the Persian Gulf has been analyzed by Rock-Eval 6 pyrolysis system. About 75 to 100 mg of the powdered cuttings from different depth of the Pabdeh Formation was heated up to 850 °C with the range of 25 °C/min in anoxic condition. The amount of released hydrocarbons, CO and CO2, was measured and the S1, S2, S3 picks were obtained.
Discussion of Results and Conclusions
In geochemical analysis, assessment of oil contamination in samples is very important as it ruins the quality of results. Based on the S1/TOC diagram, three samples had oil contamination and thus they have not been considered for the evaluations. The two diagrams of TOC/S2 and TOC/(S1+S2) demonstrated that the Pabdeh Formation has fair to excellent hydrocarbon generation potential in the studied area. Among the all samples analyzed, 10 percentage of samples are in poor, 18 percentage in fair and 72 percentage in good to excellent ranges of hydrocarbon generation potential. The S2/TOC diagram showed that the kerogens are mostly types II, III or a combination of these two types. The Van-Kerevelen diagram also confirmed that the kerogen types are II, III and II/III. Based on the kerogen types, the Pabdeh Formation could generate both natural gas and crude oil. However the maturity diagram (Tmax/HI) showed that the formation is immature and within diagenesis stage. This diagram showed that only 6.8 percentage of the samples are within oil generation window.
In general, the Pabdeh Formation in northwestern coastal part of the Persian Gulf and southern Dezful Embayment has good hydrocarbon generation potential but it is immature and has not generated hydrocarbon. Therefore another source rock should be considered for the generated hydrocarbon in the studied area.
For determining the organic facies and sedimentary environments of the Pabdeh Formation in the Persian Gulf and southern Dezful Embayment, the Jones and OI/HI diagrams were used. Based on this study, variety of sedimentary environments (B, CD, BC and C) have been involved in deposition of the Pabdeh Formation in the studied area. These ranges were mostly associated with redux to semi oxide sedimentary conditions. Different sedimentary environments and lateral facies changes are the most important reasons for wide distribution of organic facies and different kerogen types.
In addition to sedimentary environments, relative sea level changes also could have controlled the vertical facies changes in sedimentary successions. This could control the different organic facies and kerogen types of the formation with different sources from continental to oceanic environments.
Correlation between hydrocarbon generation potential, TOC, HI and kerogen type showed that the Pabdeh Formation could be divided into three (A, B and C) geochemical zones. The zones A and C, which are upper and lower parts of the formation respectively have lower amount of TOC and HI, deposited during regression. This was the time of relative sea level fall associated with kerogen type III, sourced from continental environments which could have mostly gas generation potential. However the zone B, which is middle part of the formation with brown shale lithology has higher amount of TOC and HI, deposited during transgression. This was the time of relative sea level rise associated with kerogens type I and II, sourced from marine environments which could have commonly oil generation potential.
In general the Pabdeh Formation in the southern part of the Dezful Embayment and northwestern coastal part of the Persian Gulf can be characterized as high hydrocarbon generation potential but immature source rock. However, the middle part (B zone) of the formation could be considered as a potential unconventional reservoir.