| Summary: | Bulu Formation is part of North East Java Basin that has widespread distribution especially in the North
Rembang Anticlinorium. This formation composed of larger foraminifera limestone and fossiliferous
sandy limestone which was deposited during Middle Miocene to Early Late Miocene time (N13-N15).
This paper focuses on heterogeneity of large foram organisms and their porosity, implication for
determining depositional facies. This study uses outcrop data and petrography analysis to determine
the species of large foraminifera as well as calculating visible porosity. Large foram is clustered into
three groups based on shell test character, the first group consists of Amphistegina and Miogypsina, the
second group consists of Cycloclypeus, Lepidocyclina, Operculina, Heterostegina, and plankton,
whereas the third group consists of Alveolinella, Flosculinella, Calcarina and Amphistegina. Porosity
calculation shows poor to very good classification, with value ranging from 5 to 25%.Large Foram is
used to analyze the depositional environments based on the shell shape, genus and quantity. The analysis
showed that the Formation of Bulu in the research area is deposited in open-shelf facies and toe-ofslope facies. Open-shelf facies is characterized by genus Amphistegina, Cyclolypeus, Miogypsina, and
Lepidocyclina. Toe-of-slope facies is characterized by genus that is similar to open-shelf facies but
dominated with Cycloclypeus genus. The abundance of the certain organisms could be an indication of
depositional environment of the organism.
Keywords: Carbonate, Large Foraminifera, Bulu Formation, Porosity, Stratigraphy.
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