Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Origins of Shallow-Water Carbonate Lithofacies Thickness Distributions: 1D Forward Modelling of Relative Sea-Level Control
(1) Dept. Earth Sciences, Royal Holloway University of London, Egham, United Kingdom.
(2) Energy Technology Company, Chevron, Houston, TX.
The observation that shallow-marine carbonate strata often have exponential lithofacies thickness distributions is one of the most fundamental results in carbonate stratigraphy in recent years. This is both because it is an observation that can be tested for its repeatability in outcrop and subsurface examples, and also because it raises the question of what sedimentary processes lead to the formation of particular lithofacies thickness distributions. This in turn links to the significant issue of how carbonate strata record climatic and oceanographic change through geological time.
This study applies a simple 1D numerical stratigraphic forward model of carbonate platform strata (Dougal) to investigate how relative sea-level oscillations could control lithofacies distribution. Dougal records platform-top carbonate accumulation influenced by water-depth dependent sediment production in euphotic, oligophotic and aphotic production profiles with a lag-depth controlling onset of production.
Results from single model runs highlight the issue of non-stationary behaviour where statistical properties of the strata change with elevation up the section, and show that exponential lithofacies thickness distributions can be generated from an entirely deterministic model. Results of multiple model runs (more than 27000 in total) spanning a range of production and accommodation creation rates, demonstrate that the accommodation and sediment supply do act as major, though non-linear, controls on carbonate lithofacies distribution, but significantly that lithofacies distributions also have an autocyclic control through oscillations in deposition during certain high-frequency rising limbs on the glacio-eustatic curve. In these multiple model runs only about 13% of the total runs created exponential distributions, compared to 28% in the documented outcrop examples, also suggesting that other processes, including three-dimensional process not included in this model, play an important role.
In addition to providing some understanding of the nature of lithofacies thickness distributions under varying oceanographic and climatic regimes, the findings presented here have broader implications. This is particularly true where lithofacies thickness has an impact on the performance and productivity of hydrocarbon reservoirs, such as economically-important platform and ramp interiors in both icehouse and greenhouse settings.