HIGH RESOLUTION SEQUENCE STRATIGRAPHY AND DIAGENESIS IN CARBONATE ROCKS, WONOSARI FORMATION, YOGYAKARTA: AN OUTCROP ANALOG FOR MODELING CHALKY LIMESTONE RESERVOIR DISTRIBUTION

摘要

The importance of outcrop data in the development of carbonate sequence stratigraphy and diagenesis is highlighted by a study of the Middle Miocene carbonate of the Wonosari Formation, Yogyakarta. Four carbonate sequences are studied for use as analogs to model the distribution of chalky limestone reservoirs. Depositional and diagenetic fades have been correlated allowing a model of fades development and chalky limestone genesis and distribution in response to relative sea level changes to be reconstructed for the study area. The Wonosari Formation comprises five main depositional facies as follows: (1) Reef indicated by coral framestone to bafflestone. (2) Reef Mound indicated by red-algal bindstone. (3) Near Reef indicated by branching-coral-fragment rudstone to floatstone. (4) Near Reef Mound indicated by rhodolith rudstone to floatstone. (5) Inter-reef Lagoon indicated by foraminiferal mixed skeletal packstone, wackestone to mudstone. Sequence boundaries can be continuously traced in outcrops and show different physical expressions in each sequence. Two types of sequence boundary can be identified. The first is represented by an irregular surface and unconsolidated lithoclasts and the second is represented by calcrete and paleosols. Rapid rises in relative sea level during transgressions result in retrogradational patterns. Catch-up or prograding patterns develop during highstand in relative sea level. At least four periods of relative sea level fall are recognized in the area which exposed the carbonate platform and resulted in alteration of hard limestone to porous and friable chalky limestone. The chalkification is proven to play an important role in the enhancement of secondary porosity in limestone. Vertical distribution of chalky limestone in a sequence is always bounded in its upper part by calcrete and paleosols and in its lower part by gradational changes from chalky into hard limestone. It does not develop below sequence boundaries characterized by irregular surface and unconsolidated lithoclasts.