Electrofacies- and dipmeter-aided modeling of Forbes Formation, Sacramento Valley
Conference
·
· Am. Assoc. Pet. Geol., Bull.; (United States)
OSTI ID:7151997
Outer-middle deep-sea fan turbidites of the Upper Cretaceous lower Forbes Formation were characterized by wireline-log response in three wells of the Grimes area to improve interpretation of depositional environment and relation to productive trends. The fine-grained sands and thinly interbedded siltstones and claystones challenge the log analyst, because the sands are lenticular and areally limited, and both lithology and gas zones are commonly masked by clayeyness and sand thinness. A stratigraphic Faciolog cross section shows an overall basin-filling sequence from lower Forbes deep marine distal turbidites, to upper Forbes slope deposits, to more massive Kione Formation shelf sands. The change from distal turbidites to slope deposits is characterized by a visible change in matrix grain-size composition from siltstone to largely claystone. Within the distal turbidite lower Forbes sequence, nine lithofacies of varying sandiness and cementation were identified by the Faciolog computation. This technique combined five corrected and depth-matched input curves, which were calibrated by mud logs. Rapid, objective sand counts can be made for each facies from listings of its footage in the well or interval. In this study, the lower Forbes sands are divided into six zones or turbidite episodes to simplify complex stratigraphic relationships. The cemented sandstone shells are found most commonly in the lower channel turbidite sequences. Statistical studies showed that these cemented sandstone shells have no direct relationship to gas-bearing zones. Depositionally, their relationship to the overlying and underlying sands is not well understood, but their cementation could result from a local sand-on-sand unconformity.
- Research Organization:
- Conoco Inc., Ventura, CA
- OSTI ID:
- 7151997
- Report Number(s):
- CONF-8604187-
- Conference Information:
- Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Journal Volume: 70:4
- Country of Publication:
- United States
- Language:
- English
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