Historical and Geological Perspective of Low Resistivity and Low Contrast Pay
Assignment Historical and Geological Perspective of Low Resistivity and Low Contrast Pay
1. As a petrophysicist for your company, summarize the concepts of low resistivity pay and low contrast pay for your multi-disciplinary colleagues.
Solution
- Low resistivity pay is characterized by pay zones that cause the deep resistivity logs to read from 0.5 to 5 ohm-m. This is often attributed to a combination of the shale and clay content and its high conductivity, the mineralogy, the microporosity and the bed thickness.
- Low contrast pay entails a lack of a resistivity contrast between the pay sandstones and the adjacent shales or water-bearing zones. This situation most commonly occurs when the deep resistivity logging tool encounters a zone that contains fresh formation water of high resistivity. As the salinity of the formation water decreases, the electrical pathway becomes weaker and more dispersed, and thus causes the water to become less conductive. Consequently, while the resistivity of the pay zone may not be low, the resistivity of the water zone is high enough to make it difficult to distinguish between the pay and water-bearing zones.
2. You are the project geologist for an area renowned for low resistivity and low contrast pay. Summarize the main geological causes for this to your multi-disciplinary colleagues working on the same project area.
Solution
Any combination of the following geological factors can, in certain circumstances, result in anomalously low resistivity and low contrast well log responses.
- Mineralogy: Conductive minerals, such as pyrite, glauconite, hematite and graphite, or rock fragments can have a pronounced effect on the resistivity log response.
- Distribution of the clay minerals: Clay distribution is classified as dispersed, structural, or laminated, all of which can retain significant amounts of bound water.
- Bed thickness: Some pay zones are too thin to be resolved vertically by some logging tools, especially the older tool vintages.
- Grain size: Very fine grain size can result in high irreducible water saturations.
- Structural dip: Dipping beds can produce significant excursions on certain deep resistivity logs when the orientation between the tool and the bed deviates from normal.
- Water salinity: High salinity interstitial water causes low resistivity within the pay zone, while low salinity water can cause low contrast pays.
Of all these factors, the most common cause of low resistivity and low contrast pay is the combination of thin beds containing highly conductive shales with their associated bound water, along with thin pay sandstones which are below the vertical resolution of the logging tool.
3. You are the geologist working in an area notable for low resistivity and low contrast pays. Explain to your colleagues why many potentially commercial producers have previously been plugged and abandoned.
Solution
A major reason for missing potentially commercial producers in the past was the limitation of the well logging tools available at that time. Newer logging tools take a different approach to recognizing low resistivity and low contrast pays. Important among these are the 3DeXplorer Induction Logging Tool and the Nuclear Magnetic Resonance Tool, together with their associated interpretation techniques. High resolution dipmeters and their associated visualization products have also been useful recent developments in the reservoir characterization of low resistivity and low contrast pay formations.
A secondary reason was the historical lack of awareness of the hydrocarbon producing potential of low resistivity and low contrast pays. As more technical papers and articles reviewed this phenomenon over time, the problem came to be recognized in basins all around the world, and more companies conducted drill stem testing and set production casing on low resistivity and low contrast intervals in wells.
Assessment Historical and Geological Perspective of Low Resistivity and Low Contrast Pay
1. What has had a very significant effect in disseminating interpretation techniques and case studies for evaluating low resistivity and low contrast pays?
A .Social media
B .Trade shows
C .Technical publications ✔
D .Host country government laws
2. What is the main cause of low contrast pay zones?
A .Fresh water base drilling muds
B .Shallow mud filtrate invasion
C .A zone that contains salty formation water of low resistivity
D .A zone that contains fresh formation water of high resistivity ✔
3. What is a typical deep resistivity (Rt) reading encountered in low resistivity pay zones?
A .0.05 to 0.5 ohm-m
B .50 to 500 ohm-m
C .0.5 to 5 ohm-m ✔
D .5 to 50 ohm-m
4. Historically, it has been a common practice to regard water saturations above _________ as being always non-commercial.
5. Which factors have the potential to cause low resistivity or low contrast pays? (Select all that apply.)
A .Formation water resistivity ✔
B .Microporosity ✔
C .Geological age
D .Mineralogy ✔
6. Which of the following logging tools has had most impact on low resistivity and low contrast pay interpretation?
A .Wireline formation tester
B .SP
C .Natural gamma ray
D .3D eXplorer Induction Logging Tool ✔
7. It has been suggested that up to ____________ of the world’s estimated hydrocarbon reserves are contained in laminated, low resistivity, low contrast, shaly sandstone formations.
8. What are the main mineralogical causes of low resistivity pays?
A .Salt and anhydrite
B .Calcite and dolomite
C .Clay and shale ✔
D .Quartz and aluminum
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