When a reservoir fills with petroleum, the process commonly takes about 10 Ma. Cementation of a sandstone reservoir can occur in a similarly short time. There is much direct evidence that these two processes can occur at about the same time. In petroleum-bearing sandstone reservoirs it is common to find inclusions of petroleum trapped in diagenetic minerals. The effect on reservoir porosity and permeability caused by the interaction of these two processes is dramatic. Fields with a relatively early petroleum charge, earlier that is than cementation, can be saved from the ravages of reservoir quality destruction. Fields in which the two processes were acting at the same time commonly display porositydepth gradients which are twice the regional average. If cementation beats petroleum in this 'race for space' there may be no field to produce, as the petroleum may not be recoverable from the low quality reservoir.We believe that the processes of petroleum generation/migration and cementation may share a common cause. Both processes are commonly associated with major changes within a basin: rapid burial/heating of the sediment pile and/or major faulting which changes the basin 'plumbing'.
The carbon and oxygen isotopic compositions of the Lincolnshire Limestone (Bajocian) of the East Midlands are shown to be determined largely by two end-member components: marine precipitates and later sparry burial cements. The origin of major quantities of such burial cement is controversial, the extreme possibilities ranging from calcite precipitation from meteoric groundwaters to precipitation from highly saline brines, with contrasting flow regimes.
This study demonstrates how formation-wide sampling and mapping of stable isotopic compositions of burial cements from the Lincolnshire Limestone, combined with a detailed knowledge of its burial history and associated thermal evolution, can be employed to distinguish the influence of different water types. Two 'Provinces' are differentiated with respect to the stable isotopic compositions of the latest, volumetrically most significant, ferroan burial cements.
(1) A 'Southern Province' associated with thinning of the Limestone, shallower depths of maximum burial (approx. 300 m) and light δ
18
O and δ
I3
C cement compositions;
(2) A 'Central Province' associated with deeper burial (approx. 550 m) but less
18
O- and
13
C-depleted cement compositions.
The relative isotopic depletion of the Southern Province ferroan burial cements is considered to be a function of their precipitation from meteoric waters, with a depleted
I8
O signature relative to seawater, at temperatures of approximately 24 °C.
13
C depletion is tentatively ascribed to the persistence of
13
C depleted soil-gas CO
2
in the precipitating waters.
In the Central Province, basinal waters of both meteoric and marine derivation are considered to have precipitated the late spar, with associated up-dip fluid movement at temperatures between 29 and 39 °C.
13
C depletion is believed to be controlled by the input of CO
2
derived from thermal oxidation of organic matter.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.