Suppression and Retardation of Vitrinite Reflectance, Part 2. Derivation and Testing of a Kinetic Model for Suppression

Abstract

The presence of suppressed and retarded vitrinite reflectance (VR) data introduces a number of dificulties into the prediction of hydrocarbon generation in sedimentary basins. Although the effects of suppression can be removed from measured VR values manually, a kinetic model for suppressed vitrinite maturation would enable both suppressed and unsuppressed VR values to be predicted using thermal histories derived from basin modelling. The evaluation of hydrocarbon generation fiom suppressed and unsuppressed vitrinite shows that both have similar reaction kinetics. While hydrocarbon generation involves the rupture of the bonds holding volatiles into the vitrinite structure, increases in VR are mainly produced by aromatisation and condensation reactions which take place after volatiles have been expelled. The reactions involved in hydrocarbon generation are diyerent from those responsible for increases in VR, and it is not therefore appropriate to derive kinetic models of vitrinite maturation from laboratory hydrocarbon generation experiments. During the maturation of normal (unsuppressed) vitrinite, the volatiles generated are expelled via the microporous network; the expulsion efficiency is not limited by the capacity of the microporous network. In hydrogen‐rich (suppressed) vitrinites, excess volatiles saturate the microporous network, restricting further aromatisation and condensation processes within the vitrinite, which results in suppression of VR. Kinetically, this has been modelled by using a variable pre‐exponential or “A” value. Two versions of a kinetic model of vitrinite maturation (SMod‐1 and SMod‐2) have been prepared, based on the amount of suppression predicted by HI‐VR calibration models published by Lo (1993) and Samuels son and Middleton (1998). Two case studies, involving wells Bunga Orkid‐1 (Malay Basin) and 2013–4 (Outer Moray Firth, North Sea), are discussed. Both wells contain suppressed VR values; well 20/3–4 is also overpressured and contains VR data that are both retarded and suppressed. The application of the SMod model to the wells enables heat flow histories derived from tectonic (rift) histories to be used for the prediction of VR data, although in the case of well 20/3–4, the use of a pressure retardation model was also required. Complementary evidence to support the use of the heat flow history applied to well 2013–4 is provided by palaeotemperature data obtained from diagenetic concretions.

Overpressure development and oil charging in the central Junggar Basin, Northwest China: Implication for petroleum exploration

“…The 5620 m sample from the overpressured system displays R o value of 0.77%, which seems to be abnormally low. Abnormally low R o values in overpressured systems have been observed in many basins such as the Yinggehai Basin [28,34] , the North Sea Basin [32,33,36,37] , and the Utah Basin [46] . These abnormally low R o values in overpressured rocks are attributed to overpressure retardation of organic matter maturation.…”

“…Overpressure or overpressure-related processes may retard organic matter maturation, and therefore influence the hydrocarbon generation histories of overpressured source rocks [28,[32][33][34][35][36][37] .…”

The Effects of High Pressure on Oil‐to‐gas Cracking During Laboratory Simulation Experiments

“…A hydrocarbon generation model based on the Arrhenius equation has been established for normal-pressure environments, implying that pressurerelated effects have little influence on organic matter maturation and hydrocarbon generation. However, a growing body of research shows that in a highpressure system, different maturation reactions may occur because of differences in volume expansion, causing differential retardation of the physical and chemical reactions between the products generated and the organic matter (Cecil et al, 1977;Price and Wenger, 1992;Domine and Enguehard, 1992;McTavish, 1998;Carr, 1999;2000a;2000b;2003;Hao et al, 1995;1998;Wang et al, 2006;2007a, 2007bLe Bayon et al, 2011).…”

The upper limit of maturity of natural gas generation and its implication for the Yacheng formation in the Qiongdongnan Basin, China

“…Furthermore, many maturity models which explorationists have developed are either Lopatin's Time-Temperature Indexes (TTIs) produced by calibrating the maturity with time and temperature in basins (Waples, 1980;Goff, 1983), or kinetically based (Ritter, 1984;Sweeney and Burnham, 1990;Li et al, 2008). Recently, more laboratory studies focus on the effects of pressure on VR (Price and Wenger, 1992;Hao et al, 1995) which have shown that pressure or overpressure can be incorporated into a kinetic model (Dalla Torre et al, 1997;Carr, 1999Carr, , 2000Zou and Peng, 2001), and on any other parameters capable of influencing the maturity, e.g. heating rate, lithology, hydrogen index and dry or wet conditions in laboratory pyrolysis (Khorasani and Michelsen, 1994;Huang, 1996;Lewan, 1998).…”