Gerry G. Calhoun
Consulting petroleum geologist
Midland, Tex.
The recent surge of activity involving self-sourcing reservoirs and horizontal drilling recognizes a little tapped niche in the domestic energy mix.
Such prolific pays as the Cretaceous Bakken and Austin Chalk have drawn research interest and large amounts of investment capital. Fluorescence analysis can discern movable oil-as opposed to exhausted source rock-in such reservoirs with an inexpensive test.
Other potential targets are the Cretaceous Mesaverde in the Piceance basin, Devonian New Albany shale in Kentucky, Devonian Antrim shale in the Michigan basin, and the Cretaceous Niobrara, Mancos, and Pierre formations in Colorado and New Mexico.
To insure success in this niche this key question must be answered positively: Is moveable oil present in the reservoir? Even if tectonic studies verify a system of open fractures, sonic logs confirm overpressuring in the zone, and resistivity logs document the maturity of the source, the ultimate question remains: Is movable oil in the fractures available to flow to the borehole?
IS MOVABLE OIL PRESENT?
Oil fluorescence techniques used in exploration can be modified to help answer this pivotal question. Appropriate instruments, by synchronously scanning from 250-500 manometers (nm), can identify the various aromatic compounds present in most oils. Fig. 1 (77632 bytes) shows the responses.
The four groups of aromatics are identified and quantified by their position along the wavelength scale:
- The benzene group (one aromatic ring) occurs at 290 nm;
- The naphthalenes (two benzene rings) peak at 320 nm;
- Phenanthrene-anthracene (three and four rings) peak at 350 nm;
- And complex compounds (five or more rings) peak at 410 nm. and above.
Each formation produces oil that has a unique signature in this format caused by varying amounts of aromatics in each producing zone. Source rocks, on the other hand, have signatures quite similar to each other, generally tending toward a smooth curve cresting in the 410 nm range.
COMPARING THE SCANS
A synchronous scan (Fig. 2)(50144 bytes) of Devonian oil from the Permian Basin of West Texas (dashed curve) is contrasted with a Woodford sync-scan, the generally assumed source of Devonian oil, since it immediately overlies the Devonian. The Woodford aromatics were extracted from drill cuttings.
The most radical variation in source rock sync-scans is that observed between exhausted source rock and that which appears to have movable oil retained in the drill cuttings. On the logs from a well in the Permian Wolfcamp interval in the Midland Basin, West Texas (Fig. 3)(102582 bytes), the drillstem test recovered free oil with an elevated geopressure gradient.
Representative black shale samples were removed from drill cuttings over interval one and contrasted with samples taken from interval two. Interval two closely resembles the interval that tested free oil, i.e., long travel times and high resistivity.
In Fig. 4 (48653 bytes), the dashed line shows the depleted or exhausted source rock (sample one) and the solid line, the oil-charged source rock (sample two). The area between those signatures may represent movable oil. When this over- pressured, oil-charged reservoir is located in a tectonically active area where sufficient fracturing is present, an entirely new oil play emerges. Operators with horizontal drilling experience should be particularly interested in this concept.
The Woodford source rock (Fig. 5)(51138 bytes) suggests itself as another likely candidate for exploration. It shows a condition similar to the Wolfcamp example.
Again, the solid curve signature is interpreted to be oil-charged and the dashed line is exhausted. The area between the curves is interpreted to be movable oil. Since the Woodford is up to 1 0 times richer than the Wolfcamp in total organic carbon (TOC), the "exhausted" sample may contain some movable oil, as well.
Referring back to Fig. 2 (50144 bytes) with the Devonian oil with the 350 nm. peak subdued, we see that this is the predominant hydrocarbon remaining in the source rock to be exploited. The five-plus ring aromatics (410 mn) are found in heavy, low-gravity oil while the lighter, two- to four-ring compounds are found in more mobile, higher-gravity oils.
To place Permian Basin self-sourcing reservoirs in perspective, Fig. 6 (49288 bytes) contrasts core samples from the Bakken in Antelope field, McKenzie County, N.D., with the oil-charged zone in the Woodford. The Duncan 1 Rose, in 33-15s-94w, has produced more than 300,000 bbl of oil from the Sanish-Bakken interval. The highlighted area between the Bakken (dashed line) and the Woodford (solid curve) can be interpreted as superior oil-producing potential and oil gravity in the Woodford.
TOC and S1-S2 data are closely comparable for the Bakken and Woodford. This concept has not been field tested in a borehole; however, the light aromatics identified in the "oil-charged" sync-scans closely resemble conventionally produced oils from Woodford-sourced reservoirs.
TECHNIQUE WIDELY APPLICABLE
This technique of fluorescence measurement can be applied to any potential self-sourcing pay by sampling either cores or samples from zones where log analysis indicated likely candidates (see bibliography). Ultraviolet sync-scans can highlight those intervals which still contain the lighter, more mobile oil. This simple test can keep the explorationist in the fairway of maximum recoverable oil when other factors such as orientation of open fractures, are known.
Beginning with sync-scans from producing areas, one can quickly build a library of successful and unsuccessful signatures. At about $35 per sample, several short depth intervals can be taken from a single well without breaking the G&G budget.
Drill cuttings can be analyzed during drilling so that this evidence can be included in the decision to set pipe. The key to movable oil appears to be the presence or absence of the two-, three-, and four-ring light aromatics.
This testing technique also can be used in conventional reservoirs to estimate oil gravity in a mud-log oil show.
Condensates, which are composed mostly of one- and two-ring compounds, do not fluoresce in the visible range in the standard UV box. Because we see none of the signal below 370 nm, fluorescence analysis could usefully supplement the normal evidence from mud logs.
In conclusion, fluorescence analysis may provide an inexpensive way to identify oil-charged self-sourcing reservoirs and can differentiate between productive zones and exhausted zones. It can, during drilling, guide the decision about setting casing in self-sourcing reservoirs.
BIBLIOGRAPHY
- Calhoun, G.G., Surface fluorescence method can identify potential oil pay zones in Permian Basin, OGJ, Sept. 28, 1992, p. 96.
- Dembicki Jr., Harry, Regional source rock mapping using a source potential rating index, AAPG Bull., Vol. 69, No. 4, April 1985, pp. 567-581.
- Hester, T.C. et al., Log-derived regional source-rock characteristics of the Woodford shale, Anadarko basin, Okla., USGS Bull. 1866, 1990.
- Kennicutt II, M.C., et al, Carbon isotope, gas chromatography, and fluorescence techniques applied to the North Slope of Alaska correlation, Alaska North Slope oil-rock correlation study, Magoon, L., and Claypool, G., eds., AAPG Studies in Geology No. 20, 1985.
- Requejo, A.G., et al., Utility of biological markers and aromatic hydrocarbons in correlating submarine seeps, AAPG Hedberg Research conference, in press.
- Schmoker, J.W., Determination of organic content of Appalachian Devonian shales from formation-density logs, AAPG Bull., Vol. 63, No. 9, September 1979, pp. 1,504-37.
- Webster, R.L., Petroleum source rocks and stratigraphy of the Bakken formation in North Dakota, Rocky Mountain Association of Geologists Williston basin: Anatomy of a crotonic basin, Longman, M.W., ed., 1987.
