Chronostratigraphy and Subsurface Architecture

Determining geological structures and their positions using formation ages is something geologists learn in undergrad classes. The purpose of this article is to show that in the absence of seismic data or when the seismic imaging is poor (often in sub-salt settings) stratigraphic cross sections constructed based upon the interpretation of micropaleontological data can provide information pertaining to the subsurface architecture in a given area and thus, be used as complementary tools. The analysis has to be detailed, comprehensive and integrative.

Figure 1.

Figures in this article are based upon work I have done in SE Asia during my tenure with the University of Utah. Foraminifera, nannoplankton, and palynomorph data in three wells as well as 3 2-D seismic lines of medium quality were available for this study. The focus was a single sub-basin in the area of study.

As seen in the Figure 1, a section with an age range between ~8.5 Ma and 6.5 Ma occurs only in the middle well (purple arrow) which suggests removal of the corresponding geological record from the side wells. The paleobathymetric interpretation performed with an application I developed during my tenure with the University of Utah (GomBat), indicated that all units depicted in the stratigraphic cross section have been accumulated in an outer to middle neritic environment which corresponds to outer to middle shelf. Thus, a medium range sea level fall could expose strata to sub-areal erosion. I am not a structural geologist so the interpretation below is my humble attempt to explain the geological history of the Miocene in this sub-basin.

Figure 2.

As depicted in Figure 2, following the 8.5 Ma sequence boundary normal deposition resulted in the accumulation of a package of rocks of at least 600 m but most probably more than that. Sometime before 6 Ma, due to differential subsidence, rocks in the middle of the section delineated by the chronostratigraphic profile subside about 600 m. During sea level drops at 6 and 5.5 Ma the 8.5-6.5 Ma geological record is removed from its higher elevations that were exposed for a period of time. Unfortunately none of the 3 seismic lines I had intersected the 3 wells. However, a line in which a significant depression occurs (see Figure 2) is very close to the Well 2 and a visual extrapolation of the structure shown on the seismic matches the 8.5-6.5 Ma section encountered in Well 2. 

Two yellow arrows in Figure 1, point to two units of progressively younger ages as you move from Well 1 to well 3. Most probably we are dealing with two monoclines. One eroded during sea level falls around 16 Ma and 15 Ma and the younger one during the sea level fall at 8.5 Ma.

Comprehensive analysis of micropaleontological data in conjunction with other disciplines and full collaboration with the structural geologist and the seismic interpreter can be gratifying. On a personal note, there are many out there I was working with through the years and from whom I learned a bit of seismic interpretation and a bit of structural geology. To you all, Thank You!