Stratigraphic Patterns in Carbonate Source Rock Distribution – With Special Attention to Cretaceous Intrashelf Basins of the Southern Arabian Plate

Frans S. Van Buchem
Maersk Oil Qatar AS, Doha, Qatar

The three-dimensional understanding of source rock formations is critical for accurate organic-matter volume and quality estimation as a key input parameter for basin-modeling studies. Stratigraphic heterogeneities at different scales are a well-known characteristic of marine source rocks, and occur from the millimeter to decameter scale. The two- to three-dimensional stratigraphic organization of source rock formations is, however, less known. This paper shows in three contrasting examples of carbonate platform to basin transitions, the three-dimensional architecture of carbonate source-rocks. The observed sedimentation patterns are explained involving different orders of relative sea-level fluctuations and climatic change, suggesting the potential for stratigraphic reference models. In addition, the ratio between mineral- and organic-carbon, and clays is proposed as a potential carbonate source rock classification system.

The examples are the Upper Cretaceous Natih Formation in Oman and Iran (Natih and Sarvak Formations), the Upper Devonian in western Canada (Duvernay Formation), and the Upper Carboniferous in the western United States (Paradox Formation). Different orders of cyclicity appear to have controled the distribution of organic-matter in these formations: the Cretaceous example is dominated by the third-order scale, in the Devonian example the main control is at the second-order scale, and in the Carboniferous example the fourth order is dominant. We propose that these three examples are characteristic of three climatic conditions of the earth system: the greenhouse state (the Upper Cretaceous example), the intermediate-house state (the Upper Devonian example), and the icehouse state (the Upper Carboniferous example). The internal organization of carbonate source rocks would, in that case, be predictive, and directly controlled by the known dominant sequence order in these respective states.

The ratio of (a) the carbonate fraction to (b) the organic-matter and other mineral fractions (clays, silts) shows in several examples an inverse linear relationship. This is interpreted as the result of a varying carbonate flux with stable background sedimentation of the organic-matter and mineral fraction. It implies that organic-matter production was, at times, less sensitive to short-term climatic variations than the adjacent carbonate platforms. In addition, the TOC–carbonate plot may be a useful way to characterize and compare carbonate source rocks in space and time. This would be particularly important when a relationship can be demonstrated between the mineral content and the maturation, expulsion, and migration characteristics of the carbonate source rocks.

AAPG Search and Discovery Article #90101 © 2010 AAPG Foundation Distinguished Lecturer Series 2009-2010