Classic Dipmeter Patterns For Stratigraphy
<== Arrow Plot - Disconformity and Angular Unconformity
Disconformity: This erosional surface will not be indicated by a dipmeter because the dip direction and magnitude do not change.
unconformities show up on a dipmeter as a marked change in dip
angle. The dip direction will probably change in addition.
Arrow Plot - Angular Unconformity and Drape Over Salt Dome
Angular unconformity that may be the result of folding of the formations before erosion. Similar patterns can be produced by slumping of beds below an unconformity.
dome, a structural feature often found in the Gulf Coast area,
causes formations to bend appreciably. These forces compact and
often rupture the formations involved. The dip pattern can be
in a constant direction with high dip angles. The dip direction
will normally be away from the center of the dome and will occasionally
reflect a side profile of the dome.
<== Arrow Plot - Festoon or Lenticular Cross-bedding and Tabular or Planar Cross-bedding
Lenticular cross-bedding, erratic dips occurring in definite sets. These are the dips commonly referred to as cross-bedding in sands where the dips are in all directions. The angle can be up to 30 degrees above the structural dip impressed on the adjacent formations. This difference is algebraic and dips can vary from above regional to below regional, depending upon which direction the beds have been tilted subsequent to deposition.
Tabular planar cross-bedding, most sets dip in the same direction but at different angles. Each set of cross-beds has a consistent angle and direction. Individual cross-bed sets can range in thickness to a hundred feet or more depending upon the environment of deposition.
Arrow Plot - Nonparallel Cross-bedding and Foreset Cross-bedding ==>
Wedge-shaped cross-bedding sets characteristic of aeolian deposits. Patterns of markedly different directions with good correlation and several arrows in each group; this is probably reflect wind deposition.
bedding found frequently in deltaic deposits. The arrows indicate
the direction the current was flowing at the time of deposition
if the structural dip of the region is subtracted from the dips
recorded. The minimum dips at the base of each sequence frequently
reflect the structural dip of the area as these were nearly horizontal
at the time of deposition.
<== Arrow Plot - Sand Bar and Drape Inside Channel Sand
A sand bar can often be detected on the dipmeter if the well is drilled where the borehole passes through the steep side of the sand bar. The dip increases rapidly to the top of the bed boundary, then gradually decreases through the upper beds until structural dip is again evident. This pattern of increased dip with depth and then a return to the normal trend is also characteristic of a normal fault. Here again, the need for some lithology definition is important.
The channel sand shown is an ideal type and many channels are not easily interpreted from a dipmeter. Many channel sand sequences do not exhibit the increase of dip as the bottom of the channel is approached. This increase of dip with depth as the bottom of a channel is approached probably occurs only a small percentage of the time. Many thick sequences of fluvial (water deposited) sands show no indication of this phenomenon. Many people have become disenchanted with the use of the dipmeter for interpreting stratigraphic traps because its use in channel sands has been considerably oversimplified. In deltaic sequence, the "classical" pattern is frequently found, but many other channels are not interpretable in any simple way from the dip log.
Arrow Plot - Drape Over Reef and Deep Water Turbidite ==>
Carbonate reef interpretation from a diplog is fairly straightforward for reef facies, but becomes rather complex in back reef facies. Under favorable conditions, a reef will grow upward until its organisms die due to some extreme change in environment. The reef may then be enveloped by deposits of mud. The effect of overburden and compaction will shape this new shale around the reef. The shale will reflect a pronounced change on the flanks of the reef and the shale above will reflect a lesser, but distinct amount of dip. When the reef itself is encountered the dip pattern will usually be scattered and exhibit increased dip angles. These are usually measurement of vugs, fractures, joints, etc., and should not be interpreted as a structural picture of the reef.
Graded-bedding is frequently found in beds deposited by turbidity currents. These beds are originally deposited nearly parallel with pre-existing surfaces, but may have large dips impressed upon them by orogenic movement which has taken place after deposition. The SP may not indicate the cyclic repetition if clay minerals are not more abundant in the finer grain segments of the turbidites.
Bimodal distribution of red and blue patterns indicates probable deposition in a stream channel to the East of the channel trough. The current flow is to the South.
Trimodal distribution of patterns. This pattern indicates probable deposition in a stream channel near the trough and with the current within the channel flowing to the South.
Unimodal distribution of red and blue patterns. Usually associated with barrier bar and other shoreline sediments where primary action is perpendicular to the trend of the sand body. The angles can be more dispersed if the waves hit at angles other than perpendicular or if longshore currents influence deposition to any extent. Bar trends from East to West and thins to the South.
distribution of dip patterns probably indicating a well drilled
through the crest of a barrier bar type deposit.
Page Views ---- Since 01 Jan 2015
Copyright 1978 - 2017 E. R. Crain, P.Eng. All Rights Reserved