A mathematical representation of Rock-Eval hydrogen index vs T max pro®les A. BANERJEE 1 , A. K. SINHA 1 , A. K. JAIN 1 , N. J. THOMAS 1 , K. N. MISRA 1 and KULDEEP CHANDRA 1 1 Keshava Deva Malaviya Institute of Petroleum Exploration, Oil and Natural Gas Corporation, Dehradun, India (Received 11 June 1997; returned to author for revision 4 September 1997; accepted 31 October 1997) AbstractÐDierent types of kerogens have dierent maturation paths on Rock-Eval Hydrogen Index (HI) vs T max plots. An equation: HI  1=a  expb T max  435  c; where a, b, and c are constants, can be used to de®ne HI±T max pro®les of particular source rock units. The constants a, b, and c, characterise the particular kerogen and also allow expression of its trans- formation ratio (TR) as a function of the T max by an equation: TR HI 0  a  expb T max  435=HI 0  a  expb T max  435  1; where HI 0 =1/c. The HI±T max equation constants were determined by nonlinear curve ®tting for three standard Type I, II, and III kerogens and for two geological sections: Second White Speckled (2WS) Formation, Alberta Basin and a source rock unit of Cambay Basin, India. High correlation coecients for the ®tted equations for four HI±T max data sets of four groups of the 2WS samples, segregated using Oxygen Index (OI) vs T max cross plot, and for one data set of the Cambay Basin source rock unit, show the validity of the proposed HI±T max equation. Comparison of the calculated TR and measured production index (PI) values for the 2WS samples allow identi®cation of samples that have expelled generated hydrocarbons and samples that contain unexpelled generated hydrocarbons. For the Cambay Basin example, where Rock-Eval data of silty and sandy samples were screened out, a reasonable match exists between the calculated TRs and measured production index values up to a T max value of 4458C, indicating onset of oil expulsion. The proposed HI±T max equation can thus characterise organic matter in a source rock unit with respect to its original generation potential and hydrocarbon gener- ation threshold and rate. It may also help to identify the eective source sections that have expelled generated hydrocarbons, from several potential source rocks, if present in a basin. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐRock-Eval, HI±T max curve, Second White Speckled Formation, Cambay Shale, C pro- gramme for curve ®tting INTRODUCTION The Rock-Eval pyrolysis technique helps to identify possible source rock sections and assess their qual- ity and maturity (Espitalie et al., 1977, 1985; Katz, 1983; Peters, 1986; Cooles et al., 1986). Rock- Eval T max represents the temperature of the peak rate of hydrocarbon generation. Hydrogen Index (HI) corresponds to the quantity of pyrolysed hydrocarbons relative to the total organic carbon (TOC) in the sample. HI and T max are functions of both kerogen composition and thermal history. Each dierent type of kerogen (e.g. Types I, II, and III), resulting from an interplay of OM precur- sor and paleoenvironment, has its own maturation pathline on an HI±T max plot. A mathematical ex- pression of the HI±T max pathline for a particular kerogen is desirable, because such an expression will allow characterisation of the kerogen with respect to its original generation potential (HI 0 ) and its TR pro®le. Hydrocarbon charge from dierent source rock units, at dierent locations in a basin, can thus be compared for prioritising prospects if the applicable HI±T max equations are known. In the present paper, it is ®rst demonstrated that an equation: HI  1=a  expb T max  435  c; can de®ne HI±T max curves of the standard Types I, II, and III kerogens. To further test applicability to geological data, HI±T max equation constants are determined for the 2WS Formation, Alberta Basin and a source rock section of Cambay Basin, India. Calculated TR±T max pro®les of the two example sections, using parameters of the best ®t equations, are also compared with the measured PI data. Org. Geochem. Vol. 28, No. 1/2, pp. 43±55, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0146-6380/98 $19.00 + 0.00 PII: S0146-6380(97)00119-8 43