Combined outcrops & subsurface analysis

Science

Joliet Junior College

Question Description

lease read the instructions attached, then you can use the Subsurface Facies Analysis articles :The "Subsurface Facies Analysis" article by Cant (1984) is a good overview of interpreting sedimentary facies should answer questions you may have on interpreting well logs. The Stratigraphic Sedimentologic interpretation slides will be used in lecture. Concentrate on the discussion of SP and Resistivity curves and interpretation of sedimentary sequences and subsurface facies analysis.

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1 Geoe 357 FINAL PROJECT COMBINED OUTCROP & SUBSURFACE ANALYSIS 160 points Due Monday April 20 5PM Stratigraphic Analysis and Hydrocarbon Potential of an East Kentucky Basin Assume that you have been hired as a summer intern to help expand current hydrocarbon production in an East Kentucky oil field. This oil field has produced hydrocarbon for the past 41 years, but production is now waning, and the company wishes to conduct further infill and nearby drilling for production. The field consists of 100 wells and produces from a north-south trend of sandstones within a Pennsylvanian formation. Your ultimate job is to recommend whether the company should spend its limited resources to help increase production. In order to make that recommendation you will be conducting a basin-scale analysis of the producing zone in the formation. Your analysis will utilize various types of data. The data consists of: 1) A regional geologic map of the region 2) A base map of the field showing the locations of the wells (also indicating which wells have been productive and which have been dry holes) 3) A base map showing the locations of cross sections and well logs 4) Well data spreadsheet that includes: a. top elevation and base elevation (m below sea level) of a potential reservoir interval (a depositional sequence or parasequence) in each well (normalized to meters below sea level) b. total thickness of the parasequence in each well, and c. thickness of sandstone (potential reservoir) within each parasequence in each well. 5) SP and Resistivity logs for selected wells along cross sections A to G (from which the well data were derived), and 6) Outcrop photos from an adjacent region where the formation has been uplifted and exposed. These are separated into four PowerPoint files, one for each facies sequence recognized: The following are lab products that must be submitted prior to, or at the time of, the assigned date. 1. Structure contour map: Your first task is to determine what kind of geologic structures (e.g. deformational) exist in the field and see how they correspond with where the field has been productive. To do this you need to construct a structure contour map of the top elevation for structure map. See the “top elevation” column in the well data sheet. Plot the elevations on a well site base map and contour. a. Plot all of the top elevation for structure map data points b. Neatly contour the points using a 50 m contour interval 2. Reservoir isochore map: For a full-scale analysis, you need to identify trends in the overall thickness of the formation and/or thickness of the reservoir (sandstone) and see how thickness trends correspond to where the field has been productive. For this part of the exercise, construct a sandstone-thickness map (that is, a reservoir 2 isochore map) and compare it to the location of the productive wells and to the structure contour map. A. a. Contour that map on the basemap with the cross section lines. b. Use 10 m contours c. Hints: i. assume that this in general a nearshore marine environment (the information on the facies photos are consistent with that) ii. Sediment (in general) was dispersed from the southeast towards the west and northwest) d. This map will give you some insight into: i. to the nature of the original depositional systems ii. the to controls on productive vs. dry-well locations iii. for predicting future well site locations and further production in the field. iv. Note: You will also map the four facies on this same map with neat lines and labels (item 4 below). You may eventually want to lightly color in those facies later.) 3. Facies Identification: The same Pennsylvanian formation that produces hydrocarbon from the subsurface is exposed in an outcrop belt approximately 20 km east of your subsurface project area, on the geologic map. Identify each of the four facies groups that are shown in the Facies 1, 2, 3, and 4 PowerPoint slide collections. Some of the diagnostic features for facies identification are labeled or described in the photos. Carefully observe each photo in order to identify and summarize other important properties of the facies. For each facies group prepare a list or table that contains the following: general grain size(s), terrestrial, marine, or nearshore environment, sand vs mud content, thickness of bedding, fining-upward vs coarsening-upward sequences, and any other sedimentary feature. 4. Facies map and cross-sections 1. Make sure the well dots above the logs reflect whether the wells produced oil or not. Compare them to the well dots on the maps and fill in the dots for productive wells. 2. Interpret the well-log signatures showing coarsening-upward, fining-upward, or massive sandstone sequences (use arrows like presented in the lectures) or finegrained sequences (mostly or all mud) 3. I suggest annotating the logs with values of “sandstone thicknesses”. 4. Determine where Facies 1, 2, 3, or 4 are on each of the well-log cross-section sheet, and show those boundaries between the well logs. 5. Using the well logs, draw boundaries between correlated facies units on the isopach map (the copy with cross-section lines). a. Be sure to provide neat labels on both products. b. VERY LIGHT shading of the different facies with colored pencils on this map and the cross sections may be useful. c. IF you do this be sure to be careful to not color darkly so all of your Isochore thickness pencil lines can still be easily seen! 3 5. Basin analysis. Use your isopach, structure contour, and facies data to explain the reservoir occurrence, the depositional environments, and reasons for the distribution hydrocarbon production in the East Kentucky Field. 6. Along with your maps, turn in a brief report including: a. Use multiple paragraphs, typed with double spacing. Describe the structural geology of the area, referencing the geologic map and the structure contour map. Also, include a description of the overall paleogeography (depositional setting) as represented by the set of facies types. For example, if possible discuss where was the shoreline and how did it fluctuate? What are likely trends of fluvial channels and deltaic channels? b. Name the each of the four facies and describe their properties that serve as distinguishing characteristics for environmental recognition. This list should include sedimentary texture, structures, fossils (marine vs. non-marine indicators), and any diagnostic lithologies as well as any special aspects of architectural organization. c. Interpret the reservoir facies (Fining-upward or coarsening-upward sanddominated facies) and the seal facies (mud-dominated facies) based on sedimentary interpretations. Use your knowledge from Sedimentology and Petroleum Geology classwork and the Walker (1984) article on Moodle. a. Copy and paste 2 or 3 well logs signatures for each facies that serve as examples of each of the facies groups in to illustrate your descriptions and interpretations. d. List the steps you would need to take to calculate the oil volume in the reservoir. a. Consider what additional data you would need to do these calculations. e. Drilling Recommendation a. Explain where you may propose additional drilling in the field. b. Assume that similar facies may occur at stratigraphic levels above or below this productive interval in the subsurface of eastern Kentucky. Using your structure contour data and facies maps to predict where you may explore for another accumulation. In what direction would you look for further drilling? Defend and describe your reasoning as if you were presenting this to a manager (be brief and to the point). f. List any of the references for articles or books that served as a source of information for facies identifications or other interpretations. R egional Geologic Map and Study A rea Within the K entucky B asin 5 2 Subsurface Study A rea 25 15 30 6 23 Surface Outcrops of Pennsyvanian strata Measured Section Number s Plunging A nticline N Plunging Syncline Measured Section L ocation 5 Strike and Dip of B edding T hrust Fault 25 km Structure Map Reservoir (sandstone) Isochore contour map showing the four facies Geoscience Canada, Reprint Series 1 Facies Models, Second Edition Edited by Roger G. Walker Department of Geology McMaster University Hamilton, Ontario L8S 4M1 Canada May, 1984 A fully rewritten version of the first edition, with several new contributions. Most of the papers in the first edition appeared originally in Geoscience Canada, 1976-1979, published by the Geological Association of Canada. If -I I I 297 Subsurface Facies Analysis DOUGLAS J. CANT Alberta Geological Survey Alberta Research Council 4445 Calgary Trail South Edmonton, Alberta T6H 5R7 INTRODUCTION This article will attempt to bridge the gap between "academic" sedimentol­ ogy based largely on outcrop and mod­ ern sediment studies and the tech­ niques of resource geologists who investigate sedimentary rocks in the subsurface. It is written for an audience which is unfamiliar with subsurface techniques, and is intended to be an introduction to subsurface data and procedures, particularly 1) geophysical logs, 2) cores and cuttings, 3) correla­ tion, 4) facies analysis. Seismic methods are reviewed elsewhere in this volume; readers interested in these methods are also referred to the American Associa­ tion of Petroleum Geologists Memoi r 26 (Payton, 1977), a collection of papers which summarizes a great deal of information about seismic stratigraphic analysis. Subsurface studies will probably become more important in academic facies modelling in the future. Many details of individual facies have been worked out, but we know relatively little about stacking of individual facies, or the migration of facies through time. One specific example of this is the very small number of studies documenting how fluvial point bar sequences (see "Sandy Fluvial Systems". this volume) are stacked together into meander belt sands, and how meander belt sands relate to one another. This kind of study is impossible to carry out in most out­ crop areas, but a subsurface study in an appropriate unit may succeed. DIFFERENCES FROM SURFACE WORK In many ways, subsurface data differs from the kinds of data collected from outcrops and modern sediments. Most fundamentally, subsurface data pro­ vides a differently-biased sample of the characteristics of a rock unit than does outcrop data. Drill holes and cores are concentrated in localities and zones of economic interest while outcrops pre­ ferentially expose harder, more resistent rocks occurring near the margin of a basin. Drill holes "sample" a complete section while outcrops rarely do. Some common sedimentological techniques such as paleocurrent analysis are much less applicable in the subsurface because of difficulties in obtaining data. No matter how closely spaced wells may be, data from 3 to 20 cm diameter holes cannot provide as much local information as an outcrop. However, because outcrops are in most cases two-dimensional and restricted in size. subsurface data from an extensively drilled unit may be superior for larger scale or regional studies. For example, the sizes and shapes of offshore bars are known entirely from subsurface studies (see "Shelf and Shallow Marine Sands", this volume). The variation in the most appropriate scale of investiga­ tion may be the most important differ­ ence between the two situations. GEOLOGICAL USES OF WELL LOGS Well logs are extensively used in the petroleum industry for the evaluation of fluids in rocks, but this aspect will not be covered here. The interested reader is referred to the numerous logging com­ pany manuals or other manuals such as Merkel (1979) or Asquith (1982). In most subsurface studies, geophysical logs are the fundamental source of data because virtually every oil and gas well is logged from near the top to the bot­ tom. Coal and mineral exploration drill Table 1 Log types, properties measured, and geologic uses Log Property Measured Units Geologic Uses Spontaneous potential Natural electric potential (compared to drilling mud) Millivolts Lithology (in some cases), correlation. curve shape analysis. identification of porous zones Resistivity Resistance to electric current flow Ohm-metres Identification of coals. bentonites. fluid evalua­ tion Gamma-ray Natural radioactivity - related to K. Th, U API units Lithology (shaliness), correlation, curve shape analysis Sonic Velocity of compressional sound wave Microseconds/metre Identification of porous zones. coal. tightly cemented zones Caliper Size of hole Centimetres Evaluate hole conditions and reliability of other logs Neutron Concentrations of hydrogen (water and hydrocarbons) in pores Per cent porosity Identification of porous zones. crossplots with sonic, density logs for empirical separation of lithologies Density Bulk density (electron density) includes pore fluid in measurement Kilograms per cubic metre (gm/cm3) Identification of some lithologies such as an­ hydrite, halite. non-porous carbonates Dipmeter Orientation of dipping surfaces by resis­ tivity changes Degrees (and direction) Structural analysis. stratigraphic analysis 298 Table 2 Lithology as determined by weI/logs lithology Primary log(s) Used Important Property Notes Limestone Gamma-ray Low radioactive K-content Porous limestone best distinguished from sandstone in cores or cuttings Gamma-ray Density Low radioactivity Density of 2.87 Best distinguished from limestone in cores or cuttings Sandstone Gamma-ray (SP) Low radioactive K-content Arkosic sandstone may not be identified Dolomite Shale Gamma-ray High radioactivity Conglomerate Gamma-ray Low radioactivity Anhydrite Density Density of 2.96 Halite Density Density of 2.03 Sylvite (and other K-bearing evaporites) Gamma-ray Very high radioactivity Coal Gamma-ray and sonic or density Low radioactivity, long sonic travel time, low density Argillaceous material in coal may raise radioactivity Bentonite Resistivity Low resistivity Impure ones may have high radioactivity holes may provide well log data on shal­ low rock units. Other relevant informa­ tion about subsurface methods and procedures can be found in Rees (1972), Allen (1975), Jageler and Matus­ zak (1972) and Krumbein and Sloss (1963). Best distinguished from sandstone in cores or cuttings Resistivity lohms·m2 /m) 10-13-63-21W4 ° 20 i i Spontaneous Potential (mV) 15 -H+ Deep Induction 7 2.0 Types of Logs Different types of logs, with the proper­ ties they measure and their geological uses are shown in Table 1 and dis­ cussed below. 1) Spontaneous Potential (SP) Log. This log measures the electric potential between an electrode pulled up the hole in contact with the rocks and a reference (zero) electrode on the surface. The log is measured in millivolts on a relative scale only (Fig. 1) because the absolute value of the potential depends not only Dominant Lithology Interbedded Sandstone and Sh,lIe Comments Organized into 3 CU sequences Thick, Dominantly shaly CU sequence Figure1 .. Shale Example of SP and resistivity logs. A shale line, or line of zero deflection is shown on the SP log - any deviation from this reflects por­ ous rock. Two resistivity curves are shown, one of medium depth, and one which reads deep into the formation beyond the influence of fluids from the drilling mud. The deep induction tool reads lower resistivity in por­ ous zones, probably indicating salt water saturation. The coarsening-upward (C-U) sandstones and shales are Cretaceous Mannville Group rocks, lying unconformably on Devonian Winterburn dolomite (from core). Minor Shaly CU sequence .. More Permeable .. More Resistive Sandstone Cleaner at Top Dolomite Massive, Clean 299 on the properties of the rock and inter­ stitial fluid, but also on the properties of the drilling mud. In shaly sections, the SP response is relatively constant, and it can be used to define a "shale line" (Fig. 1). Zones of permeable rock containing interstitial fluid with a salinity contrast to the drilling mud are indicated by deflec­ tions from this line. The SP log is run in most wells, and while it is not a good lithologic indicator in many areas, in others it provides the only available data which can be used. In areas of low-permeability rock such as the Deep Basin of Alberta, orthe bitumen-saturated Athabasca Oil Sand it is useless for lithologic interpretation. In freshwater-bearing units such as many Upper Cretaceous formations in Alberta, SP deflection is suppressed where low salinity drilling mud is used. However, in other areas such as the Ventura basin in California, the sand­ stones are all permeable and saturated with salt water (or hydrocarbons), with the result that the SP log delineates them very well (Hsu, 1977). Experience in an area, and calibration against cores and cuttings are the best criteria for the reliability of the SP log as a lithologic indicator. In Figure 1, the coarsening­ upward sequences are shown by pro­ gressively increasing deflection from the shale line of the SP curve, indicating more porosity/permeability upward. Gamma-Ray (API Units) Sonic Interval Travel Time (microsecs/ft.) 100 70 40 16'1.,:,'0---",,13""'0,----1'""60 Caliper (inches) 6 16 4 i 11-3-70-11W6 Dominant Lithology Comments ft 4001====~ Four Sequences Capped by Coal· 500-t===:;~ known to be Regressive Marine· to Nonmarine Cycles from Core and Shale 70°1=====~~~~===t====~c~oa~I====~~ Shale Limestone and Shale Two Sequences of Upwardly Increasing Carbonate Content 1400 14100 .. Cleaner More Shaly Limestone with Shale Interbeds .. No Trends .. Lower Velocity Higher Velocity 2) Resistivity Logs. These logs measure Figure 2 resistance of the interstitial fl uid to flow of electric current. The current flow is created either directly by electrode con­ tact, or indirectly by passing alternating currents through transmitting coils, thus inducing a magnetic field and secon­ dary currents in the rock (induction log). By lJarying the length of the tool and focussing the current, resistivities can be measured at different distances from the hole. Several reSistivity mea­ surements are commonly shown on the same track (Fig. 1) with the scale in ohm-metres, increasing to the right. Resistivity logs are used mainly for eva­ luation of the fluid content of the rocks, but are also useful for identifying coals (high resistivity), thin limestones in shaly sequences (high resistivity), and bentonites (low resistivity) (Table 2). In areas where only SP and resistivity logs are available, reSistivity logs are used for "picking" or identifying formations and correlation. Freshwater-saturated por­ Example of gamma-ray and sonic logs. Sil­ iciclastic (Lower Cretaceous Spirit River Formation) and carbonate-dominated (Dev­ onian Ireton and Leduc Formations) sections are shown. Because of space limitations in the diagram, coaly shales (with higher gamma-ray readings) are also labelled as coals, In these carbonaceous rocks, the sonic log has gone off-scale to the left and re-appeared on the right. The arrows at the bottom of the logs show the directions of variation of the important rock properties. Sequences 1 and 2 coarsen upward while sequence 3 shows no real pattern because the well is very close to the limit of this tran­ gression. ous rocks (usually very shallow) have high resistivities. Resistivity logs are therefore useful in near-surface units for separating shales from porous sand­ stones or carbonates. "cleanness" or conversely the "shali­ ness" of the rock. This property is very useful because gamma-ray patterns in many cases mimic vertical grain-size trends of sedimentary sequences, This will be discussed in more detail later in the paper. Gamma-ray lo ...
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