AAPG Geoscience Technology Workshop

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A New Normalized Maturation Scale for Marine Source Rocks


The amount of thermogenic HC generated duringmaturation (i.e., the maturation path) is a critical variable for oil and gas exploration. Maturation indices, based either on molecular proxies (e.g., 20S/(20S+20R) C29 5, 14, 17 sterane) or bulk measurements (e.g., Tmax at peak HC generation in standardized Rockeval pyrolysis), are used to delineate the maturation path relativeto HC generation stages (e.g., the so-called “oil window”). The HC generation stages are generally defined for the different types of kerogen groups (i.e., type I, II, III). This generalization often results in averaging out significant variations in HC generation within source rocks that are classified as the same type. However, source rocks classified as the same type (i.e., based on C, H and O content), such as marine oil-prone Type-II, vary in their kerogen chemistry which reflect the diagenetic environment where these rocks formed. Such variations lead to significant differences in the maturation path within a source rock type; that is, a significant variation in the amount of HC generated as a function of the degree of maturation. Here we compile maturation data of Type-II and II-S source rocks from around the world, including the Monterey, Ghareb, Toarcian, Kimmeridge and Barnett formations. These source rocks vary in: 1. The chemistry of the kerogen, 2. The initial HC potential of the immature rock, and 3. The mineral matrix (e.g., carbonaceous and siliceous). The maturation data compiled includes both natural and artificial maturation, where the latter varies in the conditions of the artificial maturation including: 1. Hydrous and anhydrous pyrolysis, 2. Relatively rapid (~6 hours) and slow experiments (~6 months) and 3. Closed and open pyrolysis systems. The compiled data show a significant variation in the maturation path leading to different estimations in the amount of HC generated as a function of thermal maturation. Here we propose a normalized maturation scale, based on the Rockeval Tmax, that considers the chemistry of the kerogen. The new normalized maturation scale brings all the data into two well-defined maturation paths that minimize considerably the variation between different source rocks. Moreover, the difference between the two well-defined maturation paths can be explained considering the molecular changes that the kerogen undergoes through the maturation process. To use the normalized scale in natural systems, an estimation of the initial HC potential of the immature source rock, as well as basic chemistry of the kerogen, is needed. Having these two variables and a Rockeval measurement of the mature sample can significantly increase the confidence of HC generation estimates.