In the studied oil field offshore Abu Dhabi, horizontal wells in the upper part of the Cenomanian "E" Formation composed of shallow-marine carbonates often encounter unexpected rocks characterized by high GR values. They have been referred to as "High-GR section (HGRS)" and thought to cause borehole instability. This paper aims to unravel the geological factors that caused the borehole instability. Coaly shale observed in cores had long been believed to be the problematic HGRS. Therefore, we conducted litho-, chemo- and biostratigraphic analyses and well log evaluation on the coaly shale and the surrounding deposits. The obtained results indicate that the coaly shale occurs as thin stratiform layers which were formed under a salt to brackish marsh environment during the deposition of the uppermost part of the "E" Formation. Although the Cenomanian/Turonian transition is the period of the Cretaceous thermal maximum when the global sea level was very high, the "E" Formation was largely exposed in the Turonian due to a regional tectonic uplift caused by the Oman ophiolite obduction onto the southeastern Arabian Plate. Coal deposition and the subsequent significant karstification in the studied field were caused by a greenhouse-induced warm and humid climate. GR log responses of the HGRS in the horizontal wells are classified into two types. One is a spiky shift that often reaches very-high GR values (>100 gAPI), and the other is a much broader shift with a flat plateau with moderately-high GR values (60–80 gAPI). The latter is always found in the wells with the borehole instability. Calibration of GR logs to the core data in the same vertical/deviated wells suggests that the intervals with very-high GR values (>100 gAPI) observed in the horizontal wells correspond to the coaly shale of the "E" Formation. Whereas, the intervals with moderately-high GR values (60–80 gAPI), which are too low for the coaly shale, are equivalent to marine shale of the overlying "B" Formation. Karstic features including chaotic cave breccia are often observed in the upper part of the "E" Formation, which suggests that caves were likely formed during the episodic subaerial exposures. Subsequently, cave collapse likely occurred during and/or after the deposition of the "B" Formation, and it could pull down the overlying "B" Formation to the reservoir level of the "E" Formation, accompanying the coaly shale. We consider that the borehole instability was primarily induced by penetrating the structurally-depressed ''B" Formation, rather than the coaly shale. This new concept is based on a comprehensive understanding of the causal link between the paleoenvironmental dynamics and the borehole instability in the studied field.
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