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Joann E. Welton1,
Scott M. Stookey2,
Richard J. Moiola1,
Harry H. Roberts3
(1) Mobil Exploration & Producing Technical Center, Dallas, TX
(2) University of Wyoming, Laramie, WY
(3) Coastal Studies Institute-LSU, Baton Rouge, LA
Abstract: Modern Mahakam delta sediments: the search for a precursor to early authigenic chlorite grain coatings
Continuous early chlorite grain coatings are recognized worldwide as an important mechanism to inhibit silica cement and preserve deep porosity in tidally-influenced depositional environments (e.g. Offshore Norway,Canada,Brazil,and Vietnam). However, little is known about how, when, and where these coatings form. To address this issue, vibracores from the muddy, mixed tide-river dominated modern Mahakam delta (Kalimantan, Indonesia) were analyzed to document the major factors controlling the development and distribution of sand and clay, and to determine how precursor grain coatings might form.
Results revealed that infiltrated clay grain coatings consisting of a mixture of chlorite, illite, and kaolinite are distributed throughtout the delta complex. Tidal processes are important because they not only effect the geometry and distribution of the sand bodies, but also bring marine waters up into the delta plain. This mixing of marine and fresh water causes the clays in suspension to flocculate, settle, infiltrate, and coat the potential reservoir sands. A significant amount of detrital mineral dissolution occurs prior to, or immediately after burial, in this tropical setting. This dissolution releases important clay-forming elements (Mg,Fe,K,Al,Si) into the system, capable of forming authigenic chlorite. Finally, SEM/CL work revealed inherited quartz overgrowths formed in a previous depositional cycle. The occurrence of inherited quartz overgrowths could easily be misinterpreted as authigenic overgrowths in the rock record.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana