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Some Problems in the Stratigraphic Analysis of Diplogs (®)(1)
Olin R. Holt (2)
In recent years, the Diplog (FOOTNOTE ®) has been increasingly used to estimate stratigraphic conditions of the formations penetrated by a borehole. In particular, the Diplog can frequently be utilized to predict the trend, direction of thickening, and movement of depositing currents in channel and barrier bar sandstones.
Channel sands tend to be more complex than interpretational literature on the Diplog implies. The original Diplog model for a channel sand resembles a distributary channel sand which does give the definitive patterns considered typical. The distributary channel sand of the Louisiana Gulf Coast is usually deposited upon relatively thick shales which are displaced by the thick distributary channel sands. The sands in the distributary channels sink into these shales and show well developed increases in dip with depth on a Diplog record through this section. For channel sands deposited on thinner shales and on more consolidated older rocks, the development of these sands does not exhibit the "normal" pattern, but generally shows a number of small channels which have moved across the valley and created a number of different sand units.
Many thick sequences of fluvial sands do not exhibit dip patterns which can be easily identified from the Diplog as being channel type sands. In some of these cases, the use of statistical analysis can help. In many instances, the determination from Diplog of the type of sand in these fluvial sequences will be difficult.
Barrier bar sands exhibit a number of different types of cross bedding within the sand unit, depending upon the relative importance of tides, longshore currents, onshore winds and storms in forming and modifying these deposits. The present interpretive techniques for this type of deposit uses differential compaction of the overlying shales as a key to the analysis.
The general interpretation of Diplogs for stratigraphic information requires the use of statistical plots. Azimuth frequency plots are especially useful in the analysis of certain stratigraphic anomalies.
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