PETROLEUM SOURCE ROCK STUDIES OF THE NON-MARINE BARAKAR SEDIMENTS, JHARIA, BIHAR, INDIA

Abstract

Commercial generation of petroleum, till recent years was thought mainly to be of marine origin. However, with the findings of substantial quantities of petroleum generated in the nonmarine sediments, from many parts of the world, the continental sediments have, of late, also become attractive for their hydrocarbon prospects. In India, the Gondwana Supergroup (Upper Carboniferous to the Jurassic/Middle Cretaceous) forms one of the two prominent nonmarine sedimentary sequences. It is also noteworthy that under similar geological setting, their equivalent nonmarine sediments in other parts of the world, notably the Patchawara and Toolachee formations of Permian age, have proved to be major source of gas in the Cooper Basin of Australia. The present study,therefore,is mainly aimed to make source rock investigations in this direction. Nonmarine Permian sediments of the Barakar Formation exposed in and around Jharia, Bihar, India, form the locale of these studies. Geologically, these areas fall in the Damuda Graben. In this basin the sedimentary fill is of over 3 km in thickness. These sediments have been stratigraphically subdivided into four units viz., Talchir, Barakar, Barren Measures and Raniganj formations. Of these, the Barakar Formation consisting of shale-coal-sand sequences, is well developed and occupies an area of about 220 sq.km. with a maximum thickness of more than 1250 m (Verma et al., 1989). The sandstone-shale-coal sequences are found to repeat vertically. As many as 50-60 cycles of such sedimentary sequences are encountered in this formation. These sediments have been intruded by mica peridotite and dolerite igneous intrusions in the form of sills and dykes. These nonmarine sediments of fluvial origin, show unidirectional cross beddings at outcrop level. It is northwesterly in the western segment, towards north in the central part and northeasterly towards or in the eastern part. Shales of the Barakar Formation found in Bhaunra, Chasnala and Phulwaritand-Maheshpur in Jharia, Bihar form the major object of source rock studies in terms of organic matter richness, quality, nature, maturation of organic matter, affect of intrusion on organic matter, clay mineral crytallinity and spectral reflectance of shales, time of generation and volumetric yield of petroleum. The total organic carbon (TOC) content of 128 samples (102 from shale and 26 from coaly shale) varies, as expected from higher values in coal rich sediments to lower values in shales. The coal rich samples have TOC values, at times, as high as 61.46%. In Chasnala area, TOC content of 13 coaly shale samples ranges from 0,94 to 57.8% with an average of 32.01% whereas the TOC of 21 shale samples in this area varies from 4.09 to 49.86% with an average of 13.19%. In Bhaunra area, the TOC of 9 coaly samples range from 13.58 to 61.46% with average of 44.83% and the shales (39 samples) in this area have wide range of TOC varying from 36.86 to 0.22% within an average of 10.97%. In Phulwaritand, the TOC range from 1.91 to 53.98% with 33.23% as an average, for 4 coaly shale samples, and 0.22 to 3.85% with an average of 1.55% for 21 shale samples was determined. Maheshpur situated northwest of Phulwaritand shows a TOC range of 0.34 to VI 0.11% with 0.21% as an average for 20 shale samples collected from this organic matter lean area. These sediments, thus, have TOC invariably more than the threshold value of 0.5% and are, therefore, of interest from the point of view of source rock investigations. TOC increases towards south and southeastern part of the area of study in Bhaunra and Chasnala. It is also important to note that the thickness of the Barakar Formation also increases towards south and southeast implying thereby higher prospects of these sediments in Bhaunra and Chasnala. Nature and Quality of Organic Matter: Even though there is sufficient organic matter in these rocks, it is important to know the nature and the type of the protopetroleum source material. Examinations of kerogen under transmitted light for 92 recovered organic matter slides from Bhaunra, Chasnala and Phulwaritand - Maheshpur areas, exhibit a mixed type of organic matter, but with domination of humic matter in which plant tissues are major contributors. Fusinite matter is also dominant in some samples. Amorphous, cloudy looking sapropelic matter is also recorded in few samples. It is, therefore, concluded that the sedimentary organic matter in these areas is mostly land derived and therefore, gas prone. Extracts of bitumens from 91 samples (49 from Bhaunra, 34 from Chasnala and 8 from Phulwaritand - Maheshpur) reveal their variation from 0.01 to 1.16% in Bhaunra, 0.01 to 0.66% in VII Chasnala and 0.02 to 0.60% in Phulwaritand - Maheshpur, indicating thereby that these rocks can be classified mainly as "rich to adequate1 in terms of source rock potential. The degree of bituminization (B) measured as extractable bitumens per unit TOC in almost all the samples of these areas, is less than 5%, which indicates that the source rocks are mainly gas prone. Organic matter mainly of humic type, or the kerogen of Type III, is considered to be gas prone. It is confirmed by low value (0.11 to 1.69) of saturate/aromatic ratio in the bitumen. Since the degree of bituminization (B) is less than 10%, it can be summarised that the hydrocarbons are syngenetic and not migratory. Gas chromatography of C15+ normal alkane fraction (satu rates) of 39 selected samples from Bhaunra, Chasnala, and Phulwaritand - Maheshpur areas was also used to determine the type of organic matter. Bivariate plot of Pristane/ nC17 against Phytane/ n C18 indicates that all the samples fall in the humic organic matter region, suggesting thereby Type III kerogen. The plot of Hydrogen Index (HI) and Oxygen Index (01) on a modified van Krevelen diagram based on pyrolysis of 100 samples (50 from Bhaunra, 34 from Chasnala and 16 from Phulwaritand - Maheshpur) confirms the quality of organic matter to be mainly the Type III kerogen. However, a few samples (8) from Phulwaritand - Maheshpur area exhibiting very low hydrogen index and comparatively high oxygen index ^are classed into Type IV kerogen. Similarly the HI- Tmax plot also confirms the Type III kerogen as the dominant constituent of the organic matter along Jl

with a little Type II and Type IV kerogens. These studies indicate that the biogenic matter ij terrestrial, deposited in peat swamps and intermediate between peat swamp and open water. These Type III Kerogens are rich in aromatic and oxygen rich compounds, and are prone to generate mainly petroleum gas on optimal maturation. Maturation of Organic Matter Kerogens generate petroleum only when they undergo optimal thermal maturation. Undercooked and overcooked kerogens do not yield hydrocarbons. Hence thermal maturation of these kerogen was determined using vitrinite reflectance (VRQ), Thermal Alteration Index (TAI), and micro- spectrofluorescence measurement. Vitrinite reflectance values measured on 93 samples (51 from Bhaunra, 34 from Chasnala and 8 from Phulwaritand - Maheshpur areas) show variations between 0.41 to 1.60% with a mean of 0.95% in Bhaunra area, 0.43 to 1.30% with a mean of 0.74% in Chasnala area, and 0.40 to 1.50% with a mean of 0.69% in Phulwaritand - Maheshpur area. Thus the kerogens found in the Barakar Formation in Bhaunra and Chasnala areas have matured to catagenetic stage implying thereby that these sediments have undergone sufficient thermal degradation .to generate petroleum hydrocarbons. As against this, the kerogens in the sediments of Phulwaritand - Maheshpur area are found mainly in the diagenetic (undercooked) stage. These observations are supported by the TAI determinations, for 92 samples (51 from Bhaunra, 33 from Chasnala and 8 from Phulawaritand - Maheshpur). IX The TAI values are centered around 2.75 showing not much variation and implying that the organic matter has undergone sufficient to adequate thermal maturation. Microspectrofluorescence measurement of organic matter was employed to determine maturity of 8 samples from Bhaunra. It is about 8.46% (Maxima) at 530 nm. for the samples unaffected by igneous intrusions. This indicates that the fluorescence has just crossed the maturity level beyond the oil window, thereby further corroborating similar deductions made from the vitrinite reflectance and TAI measurements. The foregoing investigations indicate that the Barakar sediments have sufficient organic matter, mainly of humic type. The kerogens are predominantly Type III with litfle Type II and Type IV. These have undergone maturation above the onset of oil window zone. The TOC and maturity of sediments increase towards south and south eastern part (Bhaunra and Chasnala) of Jharia basin where the sediments also attain maximum thickness (+1250 m). Thus, the areas around Bhaunra and Chasnala holding high potentials, have formed petroleum kitchens. Time of generation of Hydrocarbon Lopatin Model, based on the burial history curves and geothermal gradients was applied to determine Time - Temperature Index (TTI). The TTI of 15 at which petroleum begins to generate from kerogen of Type III was attained at about 235 million years ago before present. These sediments reached maximum maturity level corresponding to TTI of about 79 at about 200 million years ago. It is therefore inferred that these

sediments with increasing burial, had undergone maturation to produce petroleum during a period ranging from about 235 to 200 million years. Effect of Igneous Intrusions on Organic Matter These sediments have been intruded by mica peridotite. These, therefore, provide excellent opportunity for studying the effect of temperature on organic matter found in the rocks of same age, same type, at the same pressure. Because of the steep thermal gradient across the intrusion, the organic matter in the rocks close to the intrusion has been exposed to high temperatures while the organic matter farther away has been heated to only a little above the normal temperature of the country rock. Across 18 intrusions (12 dykes and 6 sills) of mica peridotite, a total of 146 samples were collected systematically at spacing varying from 5 to 150 cm, with a view to study the thermal effects on organic matter dispersed in the sedimentary rocks, in terms of variation in vitrinite reflectance (VR ) values, thermal alteration index (TAI), fluorescence, bitumen content, pyrolytic parameters, and the nature of gas chromatograms of saturates. Reflectance of vitrinite maceral indicates that the VR for the samples just at the contact with igneous intrusions, is high (as much as 6.27%) and decreases away from the intrusive bodies till it attains a ground value (where effect of intrusions is negligible) which varies from 0.40 to 1.00% in different areas. XI The thickness of this thermally affected zone varies with the thickness of intrusive body. It is generally twice the thickness of igneous body. The thermally affected zone is characterised by contact metamorphism, metagenetic and catagenetic zones, depending upon the thickness of igneous body. In general, the widths of the metamorphic, metagenetic and catagenetic zones are about 15%, 32% and 180% (nearly twice) of the thickness of the intrusive body, respectively. As the thickness of the igneous intrusions increases, the width of various maturity zones also increases. Similar observations are also made with regard to TAI and Fluorescence spectral analysis. Thermal alteration index (TAI) values are all around 2.75 except for the samples intruded by the mica-peridotites. It becomes as high as 4.5 close to the contact with the igneous body. Also, Fluorescence spectrum values are increasing towards the intrusive body. Spectral analysis, results show maximum fluorescence (maxima) of 168.44% at 730 nm wavelength in the samples at the contact with igneous intrusion and decreases to 8.4 6% at 530 nm wavelength, in the samples away from the contact. Pyrolysis of samples also, clearly indicates these effects. Tm=v values (spike of S9 peak) as high as 554°C is observed near the contact of igneous intrusion. It decreases to about 330°C (ground value) away from the contacts. It is very interesting to observe that nearly all the samples close to the contact with igneous intrusions have hydrogen index values zero or near zero, but just after a short distance comes a peak value (about 150 XII mg/g TOC or more) of HI and again it decreases to normal ground value, of 70 mg/g TOC of HI or less depending upon its TOC, where the effect of intrusions is negligible. Igneous intrusions have their effect on the extractable organic matter (Bitumen) too. At contacts or very close to intrusive bodies, volatiles appear, in general, to have been lost. This suggests that pyrolysis products from organic rich sediments at or near contacts of intrusions, were either diffused or driven away to get collected in the so called "microreservoirs". The high value of HI at a short distance from the igneous body, as observed, may be attributed to the collection of pyrolysed products in the "Microreservoirs". The gas chromatograms of these three areas also indicate that in some cases of the contacts the nCmax (maximum percentage of normal alkanes) is as low as nC18, at or near the contact and it gradually increases to ground value, of that particular bed, which is sometimes as high as nC22 or even nC24 and occasionally upto nC25« From the foregoing evidences it is seen that immediately adjacent to the intrusion, the organic matter is destroyed and only the carbon rich graphitic material remains. Also further, at the contact, the extractable material is present in large quantities but decreases with the increase in the distance from the contact and eventually reaches the values characteristic of the host shale. These intrusions of mica peridotite have thus

enhanced the maturity of organic matter and have generated additional hydrocarbons when they were emplaced in the sediments around 100 my during lower Lower Cretaceous. Genetic potential and hydrocarbon yield of shale The genetic potential of shales has been calculated using S-, and S2 peaks from the pyrolysis of shales. It varies between 0.01 to 89.48 kg of hydrocarbons per ton of source rock (kgt-1) with an average of 13.2888 kg.t-1 based on 49 samples in Bhaunra area; 0.05 to 102.4 kg.t-1 with an average of 11.698 kg. t-1 based on 34 samples in Chasnala area and 10.03 to 67.38 kg. t_1 with an average of 5.959 kg.t-1 based on 16 samples in Phulwaritand - Maheshpur area. From these results it is apparent that Bhaunra and Chasnala areas are good source rocks whereas Phulwaritand area is having a moderate source rock potential. The quantitative (volumetric) estimation of hydrocarbons generated from the shales of the Barakar Formation from these three areas in Jharia coalfield was carried out following Waples (1985). Samples for this study were selected from Chasnala, Bhaunra and Phulwaritand areas. The organic matter in the sediments is mainly terrestrial- kerogen of type III. It is matured to various levels at Chasnala and Bhaunra areas. However, it is generally immature at Phulwaritand area. Besides the maturation of organic matter during burial of sediments due to basement heat, igneous intrusions have further enhanced the maturity of these organic sediments in their vicinity. XIV In Chasnala area, volumetric estimation of hydrocarbon yield was made at three sampled locations. Of these three locations the sediments from the areaj, situated about 200 m north of Dhanbad Sindri bridge over Domohani naala (river) (Sample No. 82-93) expelled 6 million barrels of oil along with 1285174.3 million cubic metre of gas, from each cubic kilometer of source rock. However, in the other two areas, negligible amount of,gas (insufficient for expulsion) was generated. From these three locations, due to igneous intrusions 3d million barrel of additional oil and oil equivalent hydrocarbons were generated. In Bhaunra area, four locations were investigated, only two contributed. The location 0, 6 km situated NNW of Gorkhuti, generated 23.294 million barrels of oil and oil equivalent from each cubic kilometer of source rock. The other location/'about 3 km north-west of the above mentioned location, generated about 16 million barrels of oil and 1488423.7 million cubic metre of gas for each cubic kilometer of source rock, out of which 7.984 million barrel of oil along with 1190738 million cubic metre of gas expelled. In Bhaunra from these tuoo locations, an (aitroge.') additional /6-$" million barrel, of oil and £•'? x lo6 million (6 billion^ cubic metre of gas is generated from the shales in the vicinity of igneous intrusions of the area. Phulwaritand area though having a moderate source rock potential, due to their low level of maturity have generated little oil or gas. It is observed that in the vicinity of the igneous intrusions the immature organic matter has witnessed enough XV thermal maturity to generate additional oil and gas for their appropriate areas. The smaller the distance to the intrusions, the more the maturity of sediments and hence more the generation and expulsion of hydrocarbons. Also the thicker the intrusive body, more the effect, and hence more the generation of oil and gas. Clay Mineral Crystallinity as an Indicator of Maturity Physical properties of sediments as well as organic matter are changed during the process of maturation, besides changes in chemical properties. Therefore an attempt has been made to correlated crystallinity of clay mineral with the maturity of organic matter. Igneous intrusions have generated different zones of maturation of the organic matter found in the argillaceous sediments of the Barakar Formation. These intrusions must have also affected the crystallinity of clay minerals found in these sediments. if this is so, then clay mineral crystallinity may help in deciphering various stages of organic matter maturation. With this premise an attempt has been made to look into the possibility of use of clay mineral crystallinity as a maturity indicator. Clays in the shales of the Barakar Formation are kaolinite, illite and chlorite, the predominance being that of the kaolinite. Kaolinite is very sensitive to temperature changes as compared to other clay minerals. It is destroyed at temperatures of 550°c in fact, it has been destroyed in the samples very close to intrusive bodies. However, the degree of crystallinity XVI of kaolinite measured as crystallite size at half the height of its peak decreases towards the intrusive body indicating the thermal effect of the igneous bodies on the sediments as well. Using crystallite size, an attempt has been made to identify various zones of maturation. The zone of catagenesis is characterised by crystallite size between 250 A° to about 450 A". It is observed that samples in metagenetic stage have almost negligible crystallite size of kaolinite and samples in diagnesis stage or in border with catagnesis have higher average crystallite size. Values above 550 A° may be taken as indicative of a diagenetic stage. in the metamorphic zone, it is destroyed. These results may be area specific. Variation in crystallite size of kaolinite is more apparent for the thicker intrusions. Spectral Signatures of shales as Indicator of Maturity Apart from changes in the chemical properties of organic matter rich sediments, their physical properties are also expected to change through the process of maturation. Spectral responses to electromagnetic radiation, in the wavelengths 350, 400, 450 850 nm for the samples collected at varying distances from the igneous intrusions, were measured. These responses could be best studied in the Bhaunra area, as the intrusions there, are thicker and are of the order of 0.75 to 2.25 m. The reflectance increases with the increase in wavelength in the range of 350 to 600 nm and then decreases with increase in wavelength in the range from 600 to 850 nm . Also, the reflectance response for any given wavelength increases with XVII increase in the distance (of samples) from igneous intrusions and it is best exemplified at the wavelength 600 nm. The reflectance response of shales appear to be inversely related with the vitrinite reflectance. At 600 nm, the zones of metamorphism & metagenesis, catagenesis and diagenesis are maximally discriminated , and fall in the reflectances of < 3.is x 10"6 W/cm2.nm & between 3.18 and 3.25 x 10"6 W/cm2.nm, between 3.25 and 14.05 x 10-6 W/cm2.nm, and > 14.05 xlO-6 W/cm2.nm respectively. The study has demonstrated that the clay mineral crystallite size and the spectral responses to electromagnetic radiations, of shales appear to hold promise as new valuable simple tools to gauge the degree of maturity of organic matter in the argillaceous rocks. S Petroleum Propects Petroleum prospects of the area have been assessed on the basis of organic matter richness, quality and maturity of organic matter, time of generation and availability of traps. The shales of the Barakar Formation are rich in organic matter which is mainly of terrestrial type (kerogen type III) and have undergone adequate maturation to generate petroleum, mainly gas. These hydrocarbons were generated in two separate phases by two distinct sources of heat. In the first phase, petroleum hydrocarbons were generated during Permo-Triassic (235-200 my) period due to ^basement heat- while the sediments were undergoing burial. In the second stage, the igneous intrusion, specially the mica peridotite, further matured these initially matured XVIII sediments during Lower Cretaceous time (100 my), slightly away from their contacts with these sediments. These hydrocarbons were expelled as they had crossed the Mompar's (1978) threshold value of 12 m bbl. The thick pile of sediments consists of more than 50-60 cycles of shale-coal-sandstone sequences. The sandstone can act as good reservoir rocks, and shales both as source rocks and cap rocks. If the traps in the form of anticline and syncline were formed before the generation and expulsion of hydrocarbons, then these could form a good prospects. According to Mukherjee (1977) and Verma et al. (1989) the major structures in the form of folds faults were developed in two stages -the first generation of major folding took place at about 100 my during Lower Creteceous time with NW - SE axis. The second generation cross folds at about 65 m years or so during Paleocene. If this is so, then the oil/gas generated due to burial process at about 235 - 200 my had little traps available and therefore from this angle, the prospects of getting this petroleum are poor. However, since the area falls in a rift basin (Dutta 1983), some structures specially the growth faults, roll over anticlines might have been formed during or just after the deposition of these sediments. Under such conditions there may be some prospects for the petroleum generated around 235-200 my ago. In view of this, detailed basin evoluation and structural studies are required to be done to evaluate the prospects of these sediments more objectively. XIX Due to the intrusion of mica peridotite around 100 my the organic matter was matured. As the major folding, faulting and intrusion took place more or less contemporaneously, the petroleum thus generated due to intursion could have got accummulated into these trap forming structure provided theje are good cap rocks. m this connection this may be mentioned that the shales of the shale-coal-sand sequences (more than 50- 60) are if found extensive, can also serve as good cap rock.