Stable isotope stratigraphy
|Exploring for Oil and Gas Traps|
|Series||Treatise in Petroleum Geology|
|Part||Predicting the occurrence of oil and gas traps|
|Author||Robert L. Fleisher, H. Richard Lane|
The shells of some carbonate-secreting marine organisms reflect the isotopic composition of the seawater in which they live. Technical advances in micromass spectrometry and the advent of deep-ocean hydraulic piston coring have permitted, through analysis of fossil carbonate shells, a rapid, cost-effective, and precise estimation of the isotopic composition of seawater through geologic time.
Comparison of curves representing isotopic values through time can provide a basis for regional or even worldwide correlations. Although biotic and diagenetic effects must be considered when evaluating and using isotopic data, isotopic techniques can significantly augment the resolution of existing biostratigraphic zonations and provide accurate correlations within basins. Two useful stable isotopic techniques involve measuring the ratios of isotopes of oxygen (18O/16O) and strontium (87Sr/86Sr).
Oxygen isotope model
Figure 2 Correlation of a Pleistocene section between two wells in the offshore Gulf of Mexico. From Ruddiman et al. Reprinted with permission from Unocal; previously modified.
Glacial-interglacial climatic fluctuations during the late Paleogene and Neogene have been causally related to Milankovitch orbital parameters (eccentricity, obliquity, and precession). During colder glacial climates the oceans become enriched in 18O relative to 16O because the lighter 16O molecule is more easily evaporated from seawater and becomes locked on land in the form of ice. During warmer intervals the reverse is true.
Figure 1 illustrates a typical oxygen isotope record (expressed as delta oxygen-18 values) for the middle Tertiary, showing standard Oligocene and Miocene isotope stages.
This record is for the middle Tertiary and shows standard Oligocene and Miocene isotope stages (Oi 1 = Oligocene isotope stage 1; Mi 1 = Miocene isotope stage 1; etc.). Oxygen isotope data are based on analyses of benthic foraminifera recovered from cores taken at seven Atlantic and Southern Ocean sites of the Ocean Drilling Program. The curve has been fitted through the data using statistical filter and smoothing techniques.
Oxygen isotope correlation
Statistical stacking of detailed records of oceanic 18O/16O ratios, based primarily on the analysis of foraminifera from numerous deep-ocean cores and calibration to the geologic time scale, lets us construct standard oxygen isotope chronologies for the Pliocene-Pleistocene. and the Miocene and Oligocene. Thus the oceanic 18O/16O record provides a precise correlative tool based on worldwide fluctuations in climate. Isotopic analysis of well-preserved foraminifera from core or outcrop samples or from well cuttings in areas of relatively high sedimentation rate may help us recognize worldwide oxygen isotope stages. Isotope studies can be useful locally in enhancing the stratigraphic resolution of existing biostratigraphy.
Strontium isotope stratigraphy
Figure 3 Evolution of the 87Sr/86Sr ratio in seawater through the Phanerozoic. After Burke et al. Copyright: Geology.
Figure 4 87Sr/86Sr age estimates. Printed with permission of Unocal; time scale after Berggren et al.
An extensive database of 87Sr/86Sr measurements on marine carbonate, evaporite, and phosphate samples compiled at Mobil and elsewhere has permitted construction of a 87Sr/86Sr “curve” for the Phanerozoic (see illustration below). During intervals when the 87Sr/86Sr curve is relatively linear and steep with respect to time (e.g., during the Permian, Jurassic, Late Cretaceous, and several intervals within the Late Eocene to Holocene), the strontium curve can be used as a chronometer because any given ratio along the line can be associated with a unique numerical age. The accuracy of the resulting age estimates approaches ±1.0 m.y for the Cenozoic intervals.
Figure 3 shows evolution of the 87Sr/86Sr ratio in seawater through the Phanerozoic.
The strontium isotope technique is especially useful in high-latitude and shallow-water marine sections of middle to late Tertiary age where biostratigraphic zones have relatively long durations and diagnostic calcareous taxa are often absent or difficult to identify.
Figure 4 shows 87Sr/86Sr age estimates based on isotopic analysis of fossil calcareous shell material for a section of a high-latitude exploration well. Age estimates for each sample represent the mean of ages derived using the 87Sr/86Sr age relationships of DePaolo and Ingram Kopenick et al. and DePaolo. Analytical error associated with each data point yields age estimate errors of approximately ±1.0 m.y.
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