stratigraphic case histories
Formation microscanner image logs are gradually replacing standard dipmeters in many situations. The dip arrow plot, frequency azimuth plots, gamma ray, and resistivity correlation curves can be presented on a single log. This makes analysis much easier, as the visual impact of the bedding alongside the arrow plot is very powerful. The detailed dip information is ideal for stratigraphic work.

However, the computed dips can come from long interval HDT cluster and pool processing, SHDT mean square dip (MSD), continuous side-by-side (CSB) or local dip (LOC) processing. Or dips can be computed by FMS/FMI correlation or by digitizing the bedding planes seen on the image log. The choice of computation method depends on the particular requirements of the analyst. More than one computation may be required.

Unfortunately, we often work with dip and image logs created by others, and we have no control on the parameters or presentation style. We are stuck with what is in the well files, so you need to know about older as well as newer log types.

In many real cases, the processing method and parameters used are poorly described on the log heading. Try to be sure the dips presented are the dips you want or need. The illustration below shows a comparison of different processing methods on the same data. Case histories and Exercises shown later give addition examples.


Dipmeter results from various computation methods

As with any log analysis technique, calibration and control by using core and sample descriptions is very beneficial. In addition, well to well correlation and mapping can be used to help confirm stratigraphic interpretation made from dipmeter and curve shape analysis.

In addition to the Classic Examples, review of case histories often assists in consolidating analysis rules for stratigraphic interpretation of dipmeters. A number of case histories have been gleaned from the literature and the author's files to illustrate some real life examples. Because of the inordinate detail available on many logs, most of these examples have been hand drafted by the original authors for clarity.


Channel Cut and Fill
This example illustrates a channel cut and fill situation. The dipmeter on Well B indicates high north dip appearing at the base of a 500 foot sand. This dip decreases upward in a typical cut and fill pattern. The north dip indicates an east west strike to the channel, with Well B positioned south of its axis. Well C shows a loss of 80 feet of sand at the base and Well D has only 50 feet of sand remaining. Well A also shows the cut and fill pattern, but bedding dips to the south, indicating that the channel axis is to the south, between Well A and Well B.


Channel Cut and Fill


Channel Cut and Fill
This is a more complicated sequence of cuts and fills. The gross interval appears to be a series of sand and shale interbeds on the SP log. The dip data shows Sand A dipping southeast so strike is SW - NE and the axis lies SE of the well. Sand B has the same strike, but opposite dip indicates the well is SE of the axis. Sand C shows no drape, so the well penetrated near the thickest part.


Channel Cut and Fill


 Channel Cut and Fill
This is an east west cross section through the well in the previous example (Well 3). Each sand pinches out at a different elevation, so the oil water contact is different in each sand, as indicated by the high resistivity kicks in each sand. Dipmeter patterns can help identify isolated sands, and explain anomalies in oil water contacts between wells.


Channel Cut and Fill


 Barrier Bar
In Well B, the increase of dip with depth above the sand body is drape over a sand bar. Dip is 12 degrees ENE so strike is at right angles to this, or WNW - SSE. The well lies to the east of the thickest sand and the sand shales out in direction of drape (ENE). Well A found thicker sand to the west as predicted.


Barrier Bar

The dipmeter run on Well "B" exhibits the pattern of increasing dip with depth, with the maximum dip being located just above 11,000 feet. Since the maximum, dip (12 degrees ENE) is recorded at the top of a sand, this sand is a buried bar which strikes WNW-SSE and shales out to the ENE. Well "A", the West offset to Well "B", has the same sand which has thickened to 50 feet.


Unconformities
Two unconformities are shown; with no weathering pattern on the left example and severe weathering on the right. Change in dip direction and amount is pretty obvious.


Unconformities


 Foreset Beds
Four sets of foreset beds dipping SSE and a coarsening upward sequence is typical of a barrier bar, distributary mouth bar, or prograding delta front. Dips are to the SSW so sand body strikes ENE - WSW. Sand will shale out to the SSW.


Foreset Beds


 Cross-bedding
Constant dip cross-bedding in cylindrical shaped sand represents delta distributary channel fill. Sand below 7200 feet dips SE so sand body strike is NW - SE. Sand at 7000 feet dips SW so strike is NE - SW. Steep cross-bedding shows high energy, coarse grain deposits, confirmed by SP log. Regional dip removal would accentuate patterns.


Cross-bedding


Gouge
Regular dips disappear at top of gouge zone, and slight drape occurs due to differential compaction or structural deformation. The pattern is similar to a reef, but lack of carbonate rocks is the key distinguishing feature. Random dips are common in both cases below the contact.


Gouge


Shale Dome
Similar to the previous example in many ways, the shale dome shows consistent dips below the contact instead of random dips. Again, lack of carbonate differentiates this case from a reef.


Shale Dome


Reef
Drape over reef indicates direction to reef crest. Drape is to SW so reef thickens to NW. Well 16-19 has about 160 feet of pay above oil water contact. Well 4-29 missed the pay and shows drape to NE, so thicker reef could be found by whipstocking or drilling to the SW of Well 4-29.


Reef


 Channel with Foreset Bedding
This example shows the difference in detail that can be achieved by SHDT processing. Note depth scale change between different panels of the illustration. Detailed bedding analysis is shown, along with core description.


Channel with Foreset Bedding


Sand Dunes
This sand dune series is from a southern North Sea well. Constant steep foresets and decreasing bottom set dips are characteristic of dune and wadi environment.


Sand Dunes


Barrier Bar Mapping
Combining dipmeter with seismic mapping always makes for a good argument. Initial seismic mapping placed a high striking NW - SE and well control showed no hydrocarbons on downdip edges. Dipmeter in NE well showed foresets dipping SE so strike of sand body is SW - NE, at right angles to the seismic assumption. A well drilled on crest found pay and its dipmeter showed confirmation of the foresets and strike direction.


Barrier Bar Mapping

Unconformity With SYNDIP
The Cretaceous - Mississippian unconformity stands out in several ways. Resistivity shading is white for high resistivity carbonate, and Cretaceous sands and silts are shown gray or black. Bubble coding shows poor correlations and some nonplanar dips in Cretaceous contrasts with little bubble coding but numerous nonplanar dips in Mississippian.


Unconformity With SYNDIP


Deep Water Clastic With SYNDIP
This is a deep water prodelta sequence grading upward from shale to finely bedded sandstones. Long coarsening upward trends are indicated. Full clean sand is never reached. Foresets dip northwest so cleaner sand should be to southeast.


Deep Water Clastic With SYNDIP


Channel Sand With SYNDIP
SYNDIP in three wells shows variations in channel sand from well to well. Shale is well bedded and sands less so, evident from more bubble coding in the sands. Cross-bedding is often nonparallel indicating some erosion at base of each channel fill. Fining upward ramps indicate energy level decreasing as channels filled and delta prograded outward.


Channel Sand With SYNDIP

Oil Base Dipmeter in Braided Channel
Compressed scale dipmeter with computed log analysis shows thick sand section successfully logged with oil base dipmeter. Cylindrical curve shape with moderate dip spread indicates braided channel. Regional dip is ESE at about 4 degrees. Bedding in the sands is not clear on the small scale presentation. The detailed GEODIP presentation shows numerous red patterns, and an interpretation of the channels.


Oil Base Dipmeter in Braided Channel – Correlation Scale


Oil Base Dipmeter in Braided Channel – Detail Scale

   1. Abandoned channel sequence (shale with thin sandy or silty beds)
   2. Abrupt lower contact
   3. Channel lag (soft clay pebbles)
   4. Active channel sequence (massive sand and isolated events)
   5. Scour and fill deposit
   6. Longitudinal mud-channel bars (small foreset beds). Direction of transport: SSW. Channel elongation: NNE-SSW.
   7. Lateral or transverse bar (more silty); a well-developed foreset towards the top. Channel axis towards ESE.
   8. Longitudinal bar
   9. Scour and fill deposit


Sand Count Using FMS Image Examiner
Both SYNDIP and the FMS Image Examiner can be used for sand counts by using resistivity cutoffs for both the shale (low resistivity) and tight (high resistivity) ends of the spectrum. The SYNDIP approach was used. The FMS screens for choosing cutoffs and assessing results are shown here. The histogram shows the lime (tight) cutoff chosen in a known lime streak. Similarly, the shale cutoff is chosen. Remaining rock is gives a sand count of 15.4 meters or 64 percent of the analyzed interval.


Sand Count Using FMS Image Examiner


Sand Count Using FMS Image Examiner


Sand Count Using FMS Image Examiner


The Ultimate Dipmeter Analysis
By combining all dipmeter presentations, a maximum evaluation, total analysis approach to stratigraphy is achieved. A combination of DUALDIP, STRATIM, and SYNDIP with detailed core description is shown here. Note core data is on a smaller scale than dip data and is 8 feet off depth.


The Ultimate Dipmeter Analysis


The Ultimate Dipmeter Analysis


The Ultimate Dipmeter Analysis

 

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