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Geology/Marking Formation Top


Q:   Sir How We Mark The * Formation Top * In Exploratory Well ?
Q:   Sir is there specific name of Place where mud logging Unit is Placed at well site near the Shale shakers ?

Baluchistan House

Dear Waqar,

Well tops or formation tops are marked on the basis of geological well prognosis in which depth of each formation has provided with complete information i.e, depth, lithology and oil and gas shows and nature of drilling response.

The estimation of depth is based on seismic interpretation whereas lithology and petroleum information are based on the regional information. As you must know that formation is geological unit based on certain rock lithofacies, color, age etc.

While drilling, every rock sample should have monitored and examined closely under microscope by expert and experienced wellsite geologist,whenever change of lithology starts to appear wellsite geologist mark the depth of first change as the provisional well top unless it confirms by wireline log.

The wireline logs are considered a more reliable method to mark well tops as they read the true properties of the rock and reflect back on log readings unlike rock cuttings examination on well site which are mixed with overlying cuttings while drilling which may be caved in and sometimes do not provide representative sample of depth.

High GR readings lead to shale while low GR to clean sandstone, relative high density readings may give us denser rock relative to shale such as Limestone or sandstone which are further be confirmed through other logs. Well tops sometimes are also marked on the basis of the change of the drilling behavior, i.e, from fast drilling (Rate of penetration (ROP) in feet/meter  per minutes or hour  or to slow or slow to fast).

Hope this answer helps you.  


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Syed Tariq Hasany


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