Maceral characteristics and maturation of terrestrial petroleum source rocks in Thailand

Abstract

This research investigate the maceral characteristics and maturation of terrestrial petroleum source rocks in Thailand based on the organic geochemical character and organic petrography of three oilfield basins (the Fang, Phitsanulok and Suphanburi basins) and three outcrops (Na Hong basin, Ban Pa Kha sub-basin, and Mae Sot basin) were comparative studied as visible sources. The purposes of this study are to (1) determine the compositional variability of the organic matter of the source rock, (2) to identify characteristics of the maceral assemblages of the source rocks and (3) to assess the petroleum generation potential of source rocks. Geochemical analyses of source rocks were conducted using Rock-Eval analysis, Gas Chromatography, Gas Chromatography-Mass Spectrometer (GC-MS) and Total Organic Carbon (TOC) analysis. Maceral characteristics were carried out using coal petrographic microscopes. In the Fang basin, the samples were collected from Mae Sot formation between depth of 539.50 and 1,146.05 m of Fang-MS well at 10- m interval. They were deposited in a freshwater lake with moderate oxygen availability and were influenced by brackish water. TOC contents are low, and the organic matter is mainly composed of algal material (lamalginite, Botryococcus-type telalginite, fluorescing AOM, liptodetrinite). The organic matter can be classified as Types II and III kerogen. The samples are of fair to good petroleum source rock. The source rocks have a potential for mixed oil/gas and oil generation. The PI, CPI, hopane ratio and sterane ratio indicate the drilled section is thermally immature with regard to oil generation. The FANG well has been drilled to the top of the oil window which is located at a depth of about 1,100 m (VR ~0.7%Ro). In Phitsanulok basin, samples were collected from Yom, Pratu Tao and Lan Krabu formations between depths of 900 and 3,070 m of PH well at 50- m interval. They were deposited in a freshwater lake with moderate oxygen availability and influenced by brackish water. The presence of oleanane and bicadinane in some sample indicates a contribution from higher land plant organic matter. TOC contents are moderate with majority having more than 1 wt% TOC, and the organic matter is mainly composed of exsudatinite, fluorescing amorphous organic matter, liptodetrinite, laminated lamalginite, Botryococcus type telalginite, resinite and sporinite. Huminite may be present in subordinate amounts. The organic matter can be classified as Types II and III kerogen. The source rocks have a potential for mixed oil/gas and oil generation. The PI, CPI, hopane ratio and sterane ratio indicate the drilled section is early mature to mature with regard to oil generation (VR~0.40-0.66%Ro). The PH well has onset of efficient oil expulsion, which is located at a depth of about 2,800 to 3,000 m (VR~0.7%Ro). In Suphanburi basin, samples were collected from Unit A to D between depths of 1,000 and 2,840 m of SP1 well at 15- m interval and from Unit A to D between depths of 1,000 and 2,095 m of SP2 well at 5- to 10- m interval. They were deposited in a freshwater lake with moderate oxygen availability and highly influenced by brackish water. The presence of oleanane and bicadinane in some samples indicates a contribution from higher land plant organic matter. TOC contents are moderate with majority having more than 1 wt% TOC, and the organic matter is mainly composed of laminated lamalginite, liptodetrinite, fluorescing AOM, exsudatinite, Botryococcus type telalginite, resinite, sporinite and cutinite. Huminite may be present in subordinate amounts. The organic matter can be classified Type II and III kerogen. The source rocks have a potential for mixed oil/gas and oil generation. The PI, CPI, hopane ratio and sterane ratio indicate the drilled section from SP1 well is thermal maturity with regard to oil generation (VR~0.59-1.35%Ro). The drilled section from SP2 well is thermal maturity with regard to oil generation (VR~0.45-0.72%Ro). The SP1 well has onset of efficient oil expulsion which is located at a depth of about 2,100 to 2,300 m (VR ~0.70%Ro). The SP2 well has been drilled to the top of the oil window (VR ~0.70%Ro). In Na Hong basin, samples were collected from outcrops near abundant coal mine. The samples consist of mudstone, coaly mudstone and oil shale. They were deposited in a freshwater lake with low oxygen availability and influenced by brackish water. The presence of oleanane and bicadinane indicate a contribution from higher land plant organic matter. TOC contents are high with majority having more than 10 wt% TOC. The organic matter is mainly composed of lamalginite, liptodetrinite, Botryococcus-type telalginite, resinite, fluorescing AOM, sporinite, exsudatinite and cutinite. Huminite is present in high proportion and dominated by gelinite and dentrinite. The organic matter can be classified as Types II and III kerogen. The sample is good to excellent source rock properties with HI values ranging from 174–414 mg HC/g TOC. The source rocks have a potential for mixed oil/gas and oil generation. The PI, CPI, hopane ratio and sterane ratio indicate the sample is thermally immature (VR~0.40-0.49%Ro). In Ban Pa Kha sub-basin, Li basin, the samples were collected from interburden of abundant Ban Pa Kha coal mine. The samples consist of mudstone, shale and oil shale. They were deposited in a freshwater lake with moderate oxygen availability and influenced by brackish water. The presence of oleanane and bicadinane indicates a contribution from higher land plant organic matter. TOC contents are high with majority having more than 5 wt% TOC. The organic matter is mainly composed of lamalginite, Botryococcus-type telalginite, fluorescing AOM, liptodentrinite, sporinite and exsudatinite. Huminite may be present in subordinate amounts. The organic matter can be classified as Types I, II, and III kerogen. The samples are good to excellent source rock properties with HI values ranging from 308–679 mg HC/g TOC. The source rocks have a potential for mixed oil/gas and oil generation. The PI, EOM/TOC ratio, CPI, hopane ratio and sterane ratio indicate the sample is thermally immature (VR~0.36-0.40%Ro). In Mae Sot basin, samples were collected from outcrop and consist of oil shales. They were deposited in a freshwater lake with low oxygen availability. The presence of oleanane and bicadinane indicates a contribution from higher land plant organic matter. TOC contents are high with majority having more than 20 wt% TOC. The organic matter is mainly composed of lamalginite, fluorescing AOM, Botryococcus-type telalginite and liptodentrinite. Huminite is present in subordinate amounts. The organic matter can be classified as Types I and II kerogen. The samples are good to excellent source rock properties with HI values ranging from 679–831 mg HC/g TOC. The source rocks have a potential for oil generation. The PI, CPI, hopane ratio and sterane ratio indicate the sample is thermally immature (VR~0.35-0.37%Ro). In summary, all samples were deposited in freshwater with low oxygen availability for Na Hong and Mae Sot basins and moderate oxygen availability for Fang, Ban Pa Kha sub-basin, Phitsanulok and Suphanburi basins. The Na Hong basin and Ban Pa Kha sub-basin are commonly associated with coals and fluvial deposits or forest swamp deposits. The organic matter of samples from well Fang-MS, P-SK, SP1 and SP2 is similar to outcrop samples from Na Hong, Ban Pa Kha and Mae Sot basins. The organic matter is consisted of lamalginite, liptodentrinite, fluorescing amorphous organic matter, Botryococcus-type telalginite, exsudatinite, resinite, sporinite and cutinite. The organic matter is kerogen types II and III except the samples from Ban Pa Kha which is kerogen types I, II, and III and the samples from Mae Sot are all kerogen type I. The samples from FANG well are fair to good petroleum source rocks whereas samples from PH, SP1 and SP2 are poor to good petroleum source rocks but all of them have a potential for mixed oil/gas and oil generation. The samples of Na Hong, Ban Pa Kha and Mae Sot are excellent source rock. The source rocks of Na Hong and Ban Pa Kha have a potential for mixed oil/gas and oil generation where as the source rocks of Mae sot have a potential for oil generation. Samples from drilled well is more mature than the samples from surface outcrop because of deeper burial. The samples from FANG well are thermally immature to early mature where as the samples from PH and SP2 well are early mature to mature and the samples from SP1 are thermally mature. The outcrop samples are all thermally immature. The FANG and SP2 wells have been drilled to the top of the oil window. The P-SK well has onset of efficient oil expulsion which is located at a depth of about 2,800 to 3,000 m. The SP1 well has onset of efficient oil expulsion which is located at a depth of about 2,100 to 2,300 m.