It has been recognised during the past decade that the principal phase of oil generation (Vassoyevich et al.') can be identified in deeply buried Sediments, notably through the study of the diagenetic evolution of hydrocarbons. This is achieved by investigating extracts from suites of fine-grained rock samples retrieved from deep wells.
The classic diagenetic patterns for paraffins first reported comprehensively by Philippi in 1965,2 have since been observed in the subsurface by many investigators, and notably by Tissot and Pelet.3 We will briefly describe the nature of these patterns while recalling that the main cause for this diagenetic evolution is temperature modified by time: First, a sharp increase in the ratio of paraffin hydrocarbons over total organic carbon when the threshold of intense oil generation is reached.
Secondly, a shift of the mode of normal paraffin distributions from high carbon numbers (between C23 and C29) to lower numbers (between C13 and Ci9). While this shift takes place, at the threshold of intense oil generation, intermediate bimodal distributions can often be observed.
Thirdly, a smoothing out of the irregularities in the distribution of normal paraffin chains. The odd over even preferences tend to disappear as intense oil generation is approached and reached. - - [> A BARROW- I A BARROW-25 EXINITE TRIASSIC ¿i PERMIAN 0 FLINDERS SHOALS-? Besides the necessity to procure a suite of samples having undergone the right thermal history, the conditions necessary for a clear observation of these patterns are:
The absence of migrated hydrocarbons. This can be a serious problem in wells drilled on producing structures. by G. J. DEMAISON, Chevron Overseas Petroleum Inc., San Francisco, California, U.S.A. and M. SHIBAOKA, Commonwealth, Scientific and Industrial Research Organisation, North Ryde, N.S. W., Australia
The presence of hydrogen-rich kerogen in the We can demonstrate now with subsurface observations that the principal phase of oil generation is virtually non-existent, regardless of the thermal regime when the kerogen undergoing maturation is hydrogen-poor.
The subject Sediments used in this demonstration are Jurassic and Permo-Triassic shales present in the Barrow Basin, a part of the Northwest Shelf of Australia. Conventional core samples only were used to eliminate possible contamination problems.
Figure 1, a graph of H/C and O/C ratios, shows that Jurassic kerogens are hydrogen-rich and fall in diagenetic path II. On the other hand, Triassic and Permian kerogens are hydrogen-poor and fall in path III.
Sediments under investigation.
L.NEOCOMIAN & -ALGINiTE JURASSIC VITRINITE OBSERVATION-? LONG ISLAND-I ONSLOW-I SOIF o ., 251 ' I I O .10 20 .30 ATOMIC O/C Fig. 1-Compared hydrogen richness of Jurassic and Triassic Kerogens. Triassic Kerogens are hydrogen-poor.
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