Abstract

Zeolite cement zoning from heulandite via heulandite + laumontite to laumontite is found in Miocene volcaniclastic sandstones at the North Tejon Oil Field, southern San Joaquin basin, California. The zeolite transition occurs at the depth range from 2703-2743 m (present temperature range: 90-92 degrees C). Heulandite and minor smectite forms together from the dissolution of volcanic glass during early diagenesis. At depths shallower than 2743 m, early laumontite forms directly at the expense of heulandite by a reaction of the form: (1250 cc) (1470 cc)Ca (sub 3.75) Al (sub 7.5) Si (sub 28.5) O 72 .24H 2 O + 3.35Ca (super 2+) + 6.7Al(OH) 4 (super -) = 7.1CaAl 2 Si 4 O 12 .4H 2 O + 9H 2 O. heulandite laumontite. This reaction is based on the volumetrical relation between both zeolites and the absence of quartz cements during the zeolite transition. The sources of the Ca and Al appear to be partial dissolution of plagioclase and smectite occurring prior to the above reaction and incorporation of residual smectite coatings at that reaction time. Active albitization and late laumontite formation begin to occur in the laumontite-bearing sandstones at depths greater than 2743 m (present temperatures greater than 94 degrees C). This albitization reaction accompanying the late laumontite is as follows: (100 CC) (70 CC) (62 CC)Na (sub 0.7) Ca (sub 0.3) Al (sub 1.3) Si (sub 2.7) O 8 + 0.6H 4 SiO 4 = 0.7NaAlSi 3 O 8 + 0.3CaAl 2 Si 4 O 12 .4H 2 O. plagioclase albite lumontite. Clay mineral transitions from smectite via mixed-layer clay minerals (C/S or I/S) to chlorite or illite probably produced the H 4 SiO 4 necessary for the above reaction. In addition to the albitization, the source of late laumontite was derived, in part, from a mass transfer from outside. During the late laumontite formation, calcite locally forms and acts as an additional sink for Ca immediately adjacent to deformed biotite flakes.

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