Quantitative 3-D Facies Analysis of Tidal Point Bars Versus Shallow-Marine Tidal Bars: Comparative Studies of Modern and Ancient Tidal Systems for Reservoir Characterization
Shuji Yoshida1, Ron Steel2, Robert Dalrymple3, James MacEachern4, and Kerrie Bann5
1 Chiba University, Chiba, Japan
2 The University of Texas at Austin, Austin, TX
3 Queen's University, Kingston, ON
4 Simon Fraser University, Burnaby, BC
5 University of Alberta, Edmonton, AB
Tidal sandstones constitute important hydrocarbon reservoirs worldwide. Nevertheless, it remains difficult to precisely position/orient their depositional environments and their architectural elements because (1) tidal processes occur in a wide spectrum of modern proximal-distal settings, ranging from tidal rivers (e.g., 800 km inland from the Amazon mouth) to the continental slope (e.g., 1,000 m deep, offshore Indonesia), and (2) in tidal-fluvial and shallow-marine environments, tidal bars are characterized by lateral accretion, commonly with no clear differences in vertical facies profiles.
We have used the Upper Cretaceous Sego Sandstone in Utah to document facies differences between tidal-fluvial point bars and more distal deltaic-estuarine tidal bars, using architectural element analysis on 3-D outcrops. The shallow-marine bars are characterized by elongated external geometries with low-angle (c. 2-4°) accretional surfaces. The point bars are characterized by hemi-circular external geometries with high-angle (c. 5-16°) accretional surfaces, and so are large 3-D compound dunes of tidal-fluvial to marine origin. Whereas the distal bars have higher salinity indicators (as indicated by ichnofacies analysis), the lithologic and biogenic heterogeneities in both bar types vary considerably across short (10's – 100's m) lateral distances, reflecting changes in interdependent factors such as seasonal fluvial discharge, climate, and local energy conditions. The point bar outcrops have flood-dominated paleocurrent indicators and scattered brackish-water trace fossil suites, implying deposition at the distal end of a tidal-fluvial zone, close to distributary or estuarine bays. Alternatively, these point bars might represent the headward portion of abandoned distributaries as observed in modern tropical deltas today.