Evaluation of Boron as a Paleosalinity Indicator and its Application to Offshore Prospects

The validity of boron in illite as a salinity indicator has been questioned recently. Using analytical data from whole rocks, some workers argue that illite is predominantly detrital and inherits much of its boron from the parent rocks. They also believe that additional boron, absorbed at the time of deposition, is a function of soluble organic matter associated with clay minerals as well as with paleosalinity. Therefore, it has been proposed that equivalent boron variations in cyclothems, previously reported by the writer, could be related to source and organic carbon rather than to paleosalinity. Further studies of illitic clay fractions, reported in this paper, failed to reveal a significant correlation between organic carbon and boron in illite (observed boron) or the boron-K2O ratio (adjusted boron). However, equivalent boron, computed from clay-fraction analyses, may be influenced by organic carbon. Equivalent boron, as defined in earlier publications, depends on the boron-K2O ratio and the K2O concentration in pure illite. Assuming that all the K2O in a clay fraction is held in illite, the dilutent effect of organic matter must lead to inverse correlation between organic carbon and K2O. Consequently, equivalent boron which depends on the K2O concentration must be related similarly to organic carbon. Correct on for this dilutent effect leads to equivalent-boron values which correlate more closely with other geological information. Cyclothems in the lower Yoredale Formation of the upper Visean (Upper Mississippian in part) of England have been interpreted as accretion cycles related to repeated advances of deltas in a slowly subsiding basin. In Recent analogues of these cyclothems a marked reduction in salinity characterizes the initial phases of accretion, but salinity becomes extremely variable toward the end of each cycle. Near the base of Yoredale cyclothems, where paleosalinity is most predictable, equivalent boron precisely parallels the inferred salinity changes. Poor correlation between equivalent boron and inferred paleosalinity in the upper part of many cyclothems is attributed to difficulties in predicting paleosalinity from geological information, rather than to an undetected relation between equival nt boron and some other environmental parameter. Results from earlier paleosalinity investigations emphasize the exploration potential of this technique. For example, supposedly nonmarine rocks have been recognized correctly as marine; the primary depositional origin of reeflike structures has been confirmed; reef and off-reef facies have been correlated; finally, periods of maximum reef growth have been shown to coincide with periods of widespread marine transgression. Although the additional costs associated with geochemical exploration may not be justified in shallow prospects where abundant control is available, geochemical methods could increase significantly the wildcat success ratio in newly discovered oil provinces where drilling costs are high. Many offshore basins, as for example the newly discovered basins in the North Se area, are in this category.