Print this pageHydrocarbon Source Maturity Derived from Rank and Micro Constituent of Coals — A Case Study from Krishna-Godavari Basin, East Coast of India
Coal acts both as hydrocarbon source and reservoir.Rank of coal denotes the degree of maturity of organic matter. An orderly change of rank from peat to meta anthracite through brown coal and bituminous coal is the result of degree of maturity of organic matter during coalification whereby carbon content of coal increases at the expenses of its hydrogen and oxygen through natural processes. Parameters like volatile matter, carbon and hydrogen content, huminite/vitrinite reflectance (Vro) and nature of micro/petrographic constituent are considered for rank determination. Also depth of basement, burial depth, thermal conductivity of enclosing rock, intensity and duration of heat flow determine the rank.
In Krishna- Godavari Basin, scattered thin coals occur at Shallow levels from 400-1200m and occasionally from 2000-2500m depths. On the other hand Deepseated coals occur as continuous and thicker seams at depths between 3400-4400(+) m in the graben system. Both types of coals of the basin contain good amount of huminite/vitrinite. Containing fair amount of fusinite/semi-fusinite alternating with telinite/telocollinite forming a banded nature, Deepseated coals resemble to Permian Gondwana coals. Shallow level coals contain sclerotinite/fungosclerotinite along with resinite within huminite groundmass resemble to the Tertiary coals. Deepseated coals were subjected to more over burden pressure that caused clear-cut anisotropy which appears in Indian coals at Vro of 0.45% and is more pronounced above 0.90%.
Occurrence of predominantly huminite/vitrinite in groundmass with corpocollinite, phlobaphinite and fungosclerotinite in the ground matrix indicates that the source material of Shallow level coals is continental and algal/fungal origin. Such type of macerals and their relationship is prone to both gaseous and liquid hydrocarbon generation. However, a narrow window of maturity (>0.60% Vro) indicate them to be in the early stage of catagenesis. Moreover, poor thickness and scattered distribution make them unsuitable for sufficient generation of hydrocarbon and may be rated as low priority.
Deepseated coals , with Vro ranging from 1.40 to 2.17% fall within low volatile bituminous to semi-anthracite of ASTM rank. Having such rank, most of the gas from deep-seated coal might have been expelled out and stored in the overlying Chintalpudi Sandstone. This has corroborated with the good production of gas from Chintalpudi formation in the basin.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil