The Guia Formation is a limestone unit of the Araras Group, exposed in central region of South
America. They correspond to Ediacaran (622-614Ma) carbonate platform deposits, developed
in an epicontinental sea. This deposits unconformably overlies diamictite of Puga Formation
related to 635Ma Marinoan glaciation, also distributed over the metamorphic basement. The
Guia Formation, aim of study, is composed of limestones interbedded with thin layers of black
shales. The data were obtained from an analysis of a 350m stratigraphic profile in one of the
best exposition of Guia Formation in a COPACEL mine open pit in Nobres town, State of Mato
Grosso. The unit consist in six lithofacies, grouped in one facies association that records a deep
carbonate platform: bituminous lime-mudstone, massive lime-mudstone and lime-mudstone
rich in terrigenous grains, carbonate shale, carbonate breccias and deformed breccias. These
rocks were intensive affected by diagenetic process such as aggrading neomorphism, organic
matter maturation, physical compaction, spar calcite cement, dolomitization, chemical
compaction and hydrocarbon migration. Aggrading neomorphism affect the matrix, turn it to
microspar and pseudospar with hipidiotopic mosaic. The micrite rich in organic matter is
partially dissolved by fluids capable of dissolving the matrix, generating the vug pores. That
pores are cemented by spar calcite. This cement has coarse granulation, subhedral crystals,
when it fills fractures it presents crystals with rhombohedral cleavage, with two directions. In
cathodoluminescence, the cement has orange to red bright when it fills vug fracture, and it has
zoned pink bright luminescence filling pore vug. The stylolites form serrated surfaces of low
amplitude, insoluble material, composed by clays, quartz, and organic matter compound its.
Dolomitization affects the rocks in three moments: the first one is the replacement of spar
calcite filling fractures with subeuhedral dolomites in hipidiotopic mosaic and opaques minerals
inclusions, the second on is the filling by the matrix replacement with anehedral dolomites; the
third one is the formation of saddle dolomite associated with hydrocarbon. Spot analysis with
SEM/EDS, shows a large amount of Ca, ang low range of Mg, Si, S, Ca, Mn, Fe, Ni, Zn, Sr,
Cd, Ba and Pb. Reasserting that carbonate are scant changed by diagenetic/dolomitizing fluids.
These rocks are part of the Araras Petroleum System, the Total Organic Content Analysis
(TOC) indicated values between 0.04 and 0.50%, classified as a low to medium potential. The
association among the petrographic and geochemistry data suggest that the bitumen migration
occurred at 610 Ma, during thermal uplift of the Araras Intracratonic Basin, before the
deposition of the Alto Paraguai Group, which led to exposure of the Araras Group and oxidation
of the hydrocarbon, allowing the identification of type IV kerogen. Rock Eval pyrolysis, Hydrogen Index and Oxygen Index data of these rocks, do not have the potential to generate oil
or gas, and are constituted by oxidized organic matter, with immature thermal evolution.
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