EXERCISE 2: A Quick-Look Evaluation You have been asked to perform a quick back-of-the-envelope evaluation of the blocks being offered off of Somewhereia. Is there enough potential that our company should take a serious look. A check of in-house and public literature reveals that a well was drilled on the shelf, just updip from the open blocks. There is also a grid of 2D seismic lines. Both the seismic data and some information from the well are in the company records department. Figure 1 shows the location of the wells and available seismic lines. Bonanza Basin 5 6 7 8 1 2 3 4 Well A-1 Somewhereia Figure 1. Index map showing the location of the A-1 well, the 2D seismic lines, and the open blocks. 1
The well information is sparse, from a scouting service (Figure 2). However, there is a good nearshore sandstone that would form a high-quality reservoir. A good marine shale lies above the sandstone. It should provide an adequate seal. Source rocks that should be able to yield oil are present in the well, but are immature (not buried deeply enough to start generating HCs). The conclusion from the well information is that there is a potential reservoir, source, and seal. The open blocks do hold promise if: (1) the source is more deeply buried, (2) the sandstone unit extends far enough out into the basin, and (3) the sandstones maintain good porosity further out into the basin, i.e., no facies change and not too much lose of porosity with greater burial depth. 0 m Well A-1 250 m 500 m 750 m Marine Shale a Potential Seal Nearshore Sandstones a Potential Reservoir Sands contain brine No HCs Organic-Rich Shales a Potential Source Good HC source characteristics Immature not generating oil or gas 900 m Figure 2. Simplified stratigraphy for the A-1 well, based on information from scouts. 2
One of the 2D seismic lines passes near the well location. The potential source, reservoir, and seal units extend out to the open blocks, then they are cut away by a major erosional event in the early Tertiary. Although the units extend out into the basin, sedimentary facies may change downdip (e.g., the sandstone may grade into a siltstone or shale). The reservoir is also deeper and porosity decreases with depth. On the positive side, there are some rotated fault blocks that could provide large traps. Potential Source Potential Reservoir Potential Seal Well A-1 2 6 North Tertiary Figure 3. A 2D seismic lines that passes near the A-1 well location and NNE out into the basin through block 2 and block 6. 3
A study done at a university provides insights into how the Bonanza Basin formed. This study indicates that it is a divergent (pull-apart) margin with rifting during the Late Jurassic. The A-1 well is on normal continental crust, but most of the open acreage is on thinned continental or oceanic crust (Figure 4). This means higher heat flow and deeper burial for the open blocks than at the A-1 well location. This reduces the risk of an immature source rock. Oceanic Oceanic Thinned Thinned Thinned Figure 4. A map showing the types and distribution of crustal types in the area of interest. 4
You perform a quick interpretation of the top of the potential reservoir unit on the 2D seismic lines. You use a simple relationship to convert from seismic time to depth. Figure 5 shows your results (depths in kilometers). Over the open blocks, the potential reservoir varies in depth from about 500 m to 7000 m. 7.0 6.0 3.0 2.5 2.0 1.5 1.0 0.5 Somewhereia Figure 5. Top of the potential reservoir unit based on a quick interpretation of the 2D seismic lines and a simple time-to-depth relationship. (NOTE: This map is adequate for a quick-look; more careful work would be required for prospecting). 5
You have one more trick to apply to this analysis. Using the well information, you model how the source rock would mature as a function of depth and how the reservoir porosity would decrease with depth. The results of this modeling are shown in Figure 6. Source Maturity Reservoir Porosity Excellent Reservoir D E P T H 3400 m Oil Generation 4900 m Gas Generation D E P T H 3500 m 4500 m Intermediate Poor Reservoir 6400 m Figure 6. Modeling of source maturity (oil and gas generation) and reservoir porosity as a function of depth. You have a map to the top of the reservoir, but not one for the source. In the well the source is 400 m deeper than the reservoir, and on the seismic these intervals do not show much thickening or thinning. On Figure 7, indicate using color or shading where the reservoir is expected to be excellent, intermediate, and poor based on the model shown in Figure 6. On Figure 8, show where you expect oil and gas generation to be occurring based on the model shown in Figure 6. (HINT: Subtract 400 m from the depths shown in Figure 6 to compensate for the 400 m depth difference between the reservoir and the source intervals.) 6
7.0 6.0 3.0 2.5 2.0 1.5 1.0 0.5 Somewhereia Figure 7. Map of reservoir quality (porosity) based on simple modeling. Indicate on this map your preliminary estimate of reservoir quality. From Figure 6, excellent reservoir quality is expected above a depth of 3500 m. Use color, shading, or a line pattern to indicate depths above 3500 m. Intermediate reservoir quality is expected between 3500 m and 4500 m. Use a different color, shading, or line pattern to indicate this depth range. Use a third color, shading, or line pattern to indicate where poor reservoir quality is expected. 7
7.0 6.0 3.0 2.5 2.0 1.5 1.0 0.5 Somewhereia Figure 8. Map of source maturity (oil/gas generation) based on simple modeling. Indicate on this map your preliminary estimate of source maturity. This map is for the top of the reservoir, but we will use it by assuming the source is always 400 m deeper. Figure 6 indicates that above 3400 m the source is immature; it does not generate oil or gas. This area would be above 3000 m on this reservoir map (3400 400 m). Use color, shading, or a line pattern to indicate depths above 3000 m, where the source is immature. Similarly, indicate where (1) oil and (2) gas are being generated using different colors, shadings, or line patterns. QUESTIONS: 1. Are there any blocks that look promising? Which ones? 2. Are there any blocks that hold very little potential? Which ones? 8
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