Serendipity can lead to fascinating results in the life of anyone, but among geologists it can lead to new exploration and/or development opportunities.
In this instance it did!
A few years ago, I met another AAPG Foundation Trustee Associate at an ex-Texaco Christmas luncheon. During our conversation, we discovered that we both had worked in the same Gulf Coast oil field, West Columbia, though 50 years apart.
Our discussion showed that aspects of this salt structure’s growth history are poorly understood.
Figure 1. Outlines of the location of the Het Reef, representing at least the Basal Miocene-base of the ‘Marg Sand Section’, post-unconformity structures (phase 2), with the location of the younger piercement phase (phase 4) superimposed.
The most important aspect was the episodic nature of the salt movement (though hints of this history were suggested in 1929 by Carlton). This concept of episodic growth of salt piercement structures was further developed in Collins (1988) in a study of the nearby Damon Mound salt dome.
Subsequently, as Covid exploded onto the scene, a deep dive was made into the extensive historical information online and in publications, covering the first 50 years of activity at West Columbia Dome.
The initial analysis of these data made it clear that the presence of more than 400 feet of Het Reef material at West Columbia field did indeed confirm some earlier studies, indicating that:
- Early salt movement at West Columbia paused long enough for a Het Reef to grow on a seafloor high, in the shallow marine, Anahuac Sea, for several million years, during the upper Oligocene.
- The area of the Het Reef buildup was much more extensive than the current area of the later salt-piercement phase, thereby defining the sizes of the seafloor high and its supporting subsurface structures during the Oligocene (figure 1).
- This sea-bottom high remained present at least through the deposition of the Basal Miocene sands and shales, before renewed salt movement occurred.
Our evaluation of electric logs, available online or from various log libraries, plus maps and cross-sections of various vintages (1938-61), available through various government agencies, showed that the pre-Marginulina (Marg) uplift had resulted in the erosion and removal of the upper and middle Frio, and older sands and shales, over approximately the same area as that on which the Het Reef later accumulated.
Thus, the resulting major, angular unconformity covered an area much larger than that of the current piercement feature (figure 2).
The Mystery Solved
Figure 2. Chronostratigraphic Chart of the West Columbia area, showing the distribution of the Marg sands in relation to the unconformity and the underlying Frio section.
The answer to the question “where did the eroded material go?” was found within the same logs, maps and cross-sections, as above.
The eroded Frio and older material were redeposited as thin sands within the Marg shale section (locally known as the Marg Sand Section), above the unconformity. These sands are present over a large area surrounding the Pre-Marg uplift and were highly productive of hydrocarbons as early as 1938 at West Columbia field.
Due to their thinly laminated nature, they have not been fully evaluated and produced.
Several sands less than 10 feet thick, however, produced at rates of over 300 barrels per day. It is upon this Marg sand section and the overlying Marg shale that the Het Reef grew, then the conformable Basal Miocene sand and shale sections were deposited during and just after the Oligocene Anahuac Transgression.
With this more complete understanding of the episodic uplift history of salt at West Columbia, it became clear that numerous development opportunities could exist in areas of early drilling, recognizing that these “explorers” were using very old concepts and technology, and had a poor understanding of the subsurface geology.
We also evaluated the changes in economics over the early history of drilling, and even identified how major weather events, such as the hurricane of 1921, impacted subsequent activities.
More details about this study are found in our publication: “Episodic Growth History of the West Columbia Salt Dome, Brazoria County, Texas, and its Potential Impact on Hydrocarbon Accumulation” (Baie, Uphoff and Blackhall, GeoGulf 2021)
Baie, L.F., T.L. Uphoff and R.N. Blackhall, 2021, Episodic growth history of West Columbia salt dome, Brazoria County, Texas and its potential impact on hydrocarbon accumulation, In: GeoGulf Transactions v.71, p. 11-21.
Carlton, D.P., 1929, West Columbia salt dome and oil field, Brazoria County, Texas: in: Structure of Typical American Oil Fields, Volume II, American Association of Petroleum Geologists, p. 451-469.
Collins, E.W., 1988, Geology of Damon Mound salt dome, Texas: Evidence of Oligocene to post-Pleistocene episodic diapir growth: Geological Circular 88-1. Bureau of Economic Geology (Texas), 24pp.