Organic Maturity Indices for Neogene Formations in Taiwan
Date Issued
2008
Date
2008
Author(s)
Shen, Jun-Chin
Abstract
The distribution of hopanoids and alkanes in bitumen extracts of coals, coaly shales, and shales from northwestern Taiwan has been measured to assess the maturity of potential petroleum source rocks in the studied area. The C31 homohopane isomers of the studied coals and coaly shales attain equilibrium at maturity corresponding to 0.6% Ro measured in coals or 0.65% Ro in adjacent shales. A proposed parameter using hopanoid ratios reveals a more consistent trend and approaches a steady value at maturity similar to the C31 homohopane isomer ratio. The results show that the onset of the main phase of petroleum generation in northwestern Taiwan takes place at maturity corresponding to VR of 0.6% Ro measured in coals or coaly shales and probably higher VR in shales. The distribution of pristane, phytane n-C17 and n-C18 confirms the validity of the conventional maturity indicators if the samples are from the same source, but shows depositional or organic-source controls. The source influence on the phytane/n-C18 ratio appears to be larger than that on the pristane/n-C17 ratio. The phytane/n-C18 ratio is also less sensitive to maturity than that of the pristane/n-C17 ratio.he pristane/phytane ratios in the studied coals and coaly shales, in contrast to the pristane/n-C17 ratio, continue to decrease to maturities beyond the oil window up to 1.5% Ro and therefore may be applicable to high maturity sources. Thirteen desmocollinite-rich organic debrises isolated from a transgressive marine shale with a similar thermal history were characterized to study the influence of organic composition and host rocks on the variations of different maturity parameters. The results show that both maceral types of organic debrises and lithology of host rocks have little influence on the variation in vitrinite maturity. The vitrinite reflectance (0.4 to 0.55 %Ro) measured from these organic debrises, however, are slightly lower (about 0.1%Ro difference) than those from the dispersed organic matter in shale. The maturity of the organic debrises indicated by MPI-1 is consistent with that indicated by vitrinite reflectance, and the applicability of the MPI-1 indicator may be extended to maturity as low as 0.4 %Ro. The MPI-1 maturity is generally lower in samples from the siltstone than those from the shale, suggesting that the MPI-1 maturity indicator may be affected by depositional or diagenetic environments. The hopanoid index of C31 22S/(22S+22R) homohopane ratio indicates a maturity corresponding to the vitrinite reflectance measured from organic debrises. The maturities of the organic debrises indicated by three parameters (%Ro, MPI, and hapanoid ratio) are in good agreement and that the source rocks in the studied formation are very close to the onset of oil generation. The ratios of cadalene/ retene and fuoranthene/ pyrene vary significantly with the lithology: higher in samples from the siltstone than those from the shale. oals from different formations in Northwestern Taiwan were treated with hydrous pyrolysis to get its genetic potential. Genetive volume of gas from these samples were found in this study. It can be easily calculated by hydrogen index (C1 =1.5653(HI)0.2194，C2+ = 0.4869(HI)0.3126). C1/C2 increase with hydrogen index too(C1/C2 = 8E-0.7(HI)2- 0.0017(HI)+ 4.2356 ).
Subjects
Coal
Shale
Biomarker
Vitrinite
Organic Maturity
Sedimentary environment
SDGs
Type
thesis
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