Brooks J., Fleet A.J. (eds.) Marine Petroleum Source Rocks

Blackwell Scientific Publications, 1987. — 433 p.
Geochemical conferences have become a frequent feature of the earth sciences scene. There are good reasons for this--the wide range of chemistry involved across the spectrum of sediments, the fascinating problems of the origin and diastrophism of the organic element in the sediments and the light which the complex chemistry can throw on the thermal, temporal and pressure history of the rocks since their initial deposition. Added to this is the automated equipment for production of multiple analyses both in the lab and on the rig and the availability of computer programs to evaluate the results, and of course, the fundamental importance of the subject in the search for economic hydrocarbon accumulations. It is the last mentioned factor which has loosened purse strings and justified the very rapid build up of practitioners of the science.
However, this volume is the proceedings of an unusual event--a joint meeting of two Geological Society groups, one of which deals with marine studies and the other with petroleum geochemistry. This meeting is an indication of the extent to which these two very different fields have interests in common, and the scale of advance resulting from recent discoveries especially in the realm of deep ocean drilling.
The first two parts deal largely with the environment of source rocks in oceans and on continental margins, with an emphasis on concepts and methods in dealing with the organic-rich sediments generated during world-wide 'anoxic events'. The third part has a stronger stratigraphical bias, but still deals with the fruitful ground where oceanography and organic geochemistry are jointly concerned. The spatial relations between the contemporaneous anoxic muds on continental shelves and deep ocean circulation are a particularly interesting aspect of the joint studies.
As a geologist who has had a career in petroleum exploration I feel I should express concern about the regional validity of sampling based on industrial activity, a warning which is as relevant to offshore as to landward operations. Since hydrocarbons are lighter than formation water it is inevitable that they are sought in structural highs, whether in anticlinal crests, fault blocks or in stratigraphic traps. It is consequently rare for exploratory boreholes to plumb the depths of modern synclines, and well samples of a given formation are likely to be misleading as to the total basin-wide fill in terms of thermal history. Just as a reasonable reservoir rock on a high may be found to deteriorate for physico-chemical reasons down flank into a basin, so a source rock found to be immature in every (anticlinal) test well could have been effective a thousand metres or more lower in adjoining synclines. It is clearly important that 'whole basin studies' should be attempted wherever possible.
This historical factor is now increasingly kept in mind by geochemists but there is a corollary--that source sediments now too deep to generate oil in a temperature/pressure regime which isundoubtedly gas-prone, are likely in the normal course of basin subsidence during the geological past to have been at depths appropriate for oil generation. This is (hopefully) obvious to the geologist; the chemists will perhaps forgive my underlining this important and inescapable fact. Linked with this may I plead for more precision in references to source-rock potential. It is rare to find clearly specified whether a given sediment has been or will in future be a good source for generation of hydrocarbons. A shale still rich in organic content may never have served as a source or one totally barren may have been stripped. The time element, the geological history, should be intrinsic in the definition, otherwise use of the terms may be quite misleading in relation to a given problem of petroleum generation, particularly when the data are used directly by the non-technical. There are many questions in petroleum and oceanic geochemistry which are still unsolved; but this volume should considerably advance our knowledge even if we do not arrive at final conclusions to many of them.