Using Well Logs (e-logs) in the Petroleum industry

1. Using Well Logs (e-logs) in the Petroleum industry Earth Science World

2. Context • First exposure to well logs - petroleum industry. • Well logs are proxies for stratigraphic sections. • Identify lithology. • Represent stratigraphic sections. • Lithostratigraphic correlation. • Concepts • Porosity & permeability re-visited. • Rocks contain fluids – salty water or petroleum. • Density stratification of fluids in rocks. • Depositional models paramount in petroleum industry. • Practicality of sedimentary geology ---> jobs. • Materials • Borowski - SERC on-line materials. • 1-inch well logs purchased from a log broker, Cambe.

3. Exercise components • Lithology via well logs – shale vs. unconsolidated sand. • Formation fluids – brine vs. petroleum (oil). • Reinforces concept of porosity and permeability. • Lithostratigraphic correlation using well logs as proxy. • Concept of oil/water contact – density stratification. • Calculate elevation of strata – datum, sea level. • Structural cross section & structural trap. • Test concept of horizontal oil/water contact. • Necessity for geologists and use of depositional models in petroleum industry. Let’s run through the exercise!!

4. Chandeleur Sound Block 25 Field • St. Bernard Parish, LA • BB Sand - deltaic EODs • 40 MMBO Venice Hyne (2000) BB Sand (Tex. “L”) ~ 9 Ma John et al. (2003)

5. Base map

6. Interpreting logs • SP –spontaneous potential • Passive tool • Voltage between sensors High (+) – impermeable - shale Low (-) – permeable – non-shale • Shale baseline – connect far- right readings • Readings left of baseline • Admixtures of coarser grains w/ shale • Interbedding of sand & shales – different thickness SP shale baseline deep shallow amplified

7. Resistivity • Deep- & shallow-reading curves (sensor spacing) • Amplified curve • Rock fluid content • Water in pore spaces with increasing residence time - increase [ dissolved ions ] • Brines – high conductivity – low resistivity Low resistivity – brine – left High resistivity – HC? – right SP deep shallow amplified

8. Depth • 1 inch = 100 feet • 10 divisions each 100 feet • each division = 10 feet • Interval 5123’ – 5200’ MD • Dominant lithology = unconsolidated sand • Interbeds of sand & shale • SP curve kicks left away from shale baseline

9. Interval 5123’ – 5200’ MD • Deep (left) and shallow (right middle) read low R (high conductivity) • Consistent with brine

10. Defining the reservoir sand • BB sand & BB marker (EODs) Atlantic S/L 4542 #14 Marine shale – delta avulsion Sheet sand – delta subsidence Crevasse splay Delta lobe Distributary mouth bar

11. Matching well log curves • Find the correlation in the #4 well, using information from the #14.

12. curves • Do this correlation for all wells in the cross section.

13. Identifying brine and petroleum • High resistivity zone at top of BB Sand • Higher resistivity is inconsistent with brine, but consistent with oil (low conductivity). • Bottom of high R is at ~5285 MD. • Brine occurs below – low resistivity = high conductivity. • Top = 5272 MD, bottom = 5285 MD Thickness = ~13’ oil Atlantic S/L 4542 #4 High R

14. Color-coding lithology and fluid content Atlantic S/L 4542 #4 sand shale oil water yellowbrowngreenblue

15. Correlating the reservoir across the field • Well log correlation mimics lithostratigraphic correlation.

16. Correlated cross section

17. Petroleum patterns • Oil always occurs atop salty water in the field wells. • This occurs within pore spaces of the reservoir rock due to density difference between oil and water roil < rwater Atlantic S/L 4542 #4

18. Petroleum patterns • Pore space in rocks is filled either with water, cement (i.e., calcium carbonate, CaCO3), or petroleum. • Almost always water is first within pore spaces and must be displaced by migrating oil. • This process can be modeled by thinking of an upside-down bowl within a tub of water that is injected with salad oil. • Thought experiment or demonstration? Horizontal oil/water contact

19. Calculating elevation IRT sea level Kelly bushing (KB) – measured depth = elevation IRT elevation target sea level Atlantic 4542 #4 25’ – 5287’ = 5262’ subsea O/W contact Now determine the bottom of oil in each well and determine its subsea elevation, showing your calculation at the bottom of each well log on the cross section.

20. Testing a hypothesis • Given our experiment, the oil/water contact should be horizontal within Chandeleur Sound Block 25 Field. • This means that the contact should be at the same elevation. • Is it? Look most closely at the 4542 #4 well and compare its oil/water contact to that of the #3 and 4545 #4 wells. • Give plausible reasons why this is or isn’t so.

21. Testing a hypothesis • Look most closely at the 4542 #4 well and compare its oil/water contact to that of the #3 and 4545 #4 wells. 4542 #4 4542 #3 4545 #4 ~ -5262’ ~ -5270’ ~ -5272’ oil/water contact elev. • Elevation of oil/water contacts differs by at least 10’; maximum is ~ 33’ (#1 : #4 wells).

22. Testing a hypothesis • Give plausible reasons why the contacts may be different. • E.g., different compartments with sand reservoir.

23. Need for geology & geologists in the petroleum industry • The vertical (stratigraphic) and lateral distribution of permeable reservoir rock is dependent upon the depositional environment of that rock. • Use depositional models to assess and predict: • reservoir quality (f & k) • reservoir thickness • reservoir compartments. All figures after Reading (1978)

24. Summary • Serves as an introduction to well logging, proxy interpretation, & the petroleum industry. • More advanced exercise concerning detailed e-log interpretation loaded for this workshop. • My Petroleum Geology materials also available as handouts. • Please do hesitate to contact me concerning improvements (w.borowski@eku.edu). www.icdp-online.org www.logwell.com

25. Completed cross section - left

26. Completed cross section -right

27. Completed cross section