Organic Facies Type and Hydrocarbon Potential of Carbonates in Majiagou Fm. Lower Ordovician in Ordos Basin

Key words: 
• carbonate /
• organic facies /
• hydrocarbon potential /
• O rdos Basin

Abstract: Lower Ordovician carbonate in the Ordos basin is characteristed by its low organic matter content and high maturity, and its low residual TOC (total organic carbon) value can be not only regarded as its own initial feature but also contributed to the high maturity and high hydrocarbon potential. So whether it can be regarded as main source rock of the weathering-crust gas reservoir beneath the Upper Carboniferous/ Lower Ordovician parallel unconformity is still debatable. In this paper, the authors studied the hydrocarbon potential of Ordovician carbonates from the view of organic facies. Geochemical feature of different depositional environments includes: (1) Over 600 sample shows that carbonate formed in continental slope has a maximum average TOC value of 0.26%wt, then is that formed in gypsum lagoon (0.22%wt) and gypsodolomitic flat (0.21%); those deposited in dolomitic flat and open sea have lower TOC content (0.15%wt and 0.16%,respectively). TOC in carbonates formed in beach is minimum (0.15%). Even if taking the decrease of organic matter during maturity into account, the initial TOC value in the carbonate in its low-mature stage is still as little as in organic C～D proposed by Jones. (2) H/C and O/C atomic ratios of 20 kerogen samples all drop on the right part of the Van Krevelen chart. High O/C value indicates that the low H/C of organic matter in the carbonate is the feature during its formation instead the result of hydrocarbon generation, low H/C ratio also indicate a poor organic facies. (3) Mean Fe³⁺ /Fe²⁺ value is the highest of 33 carbonate samples formed in beach (8.55), then 5 open sea carbonate samples (0.78), 3 gypsum lagoon and gypsodolomitic plat carbonate samples (0.36). They are all higher than those in hydrocarbon generation or reservoir strata of Carboniferous-Permian (mean 0.05 of 12 samples) and Ordovician weathering crust (mean 0.07 of 15 samples), showing the lack of hydrocarbon and high oxidation potential. (4) Lithology and fossil feature also show a bad condition for the preservation of organic matter (oxidation and consumption of fauna). So most depositional environments of Lower Ordovician carbonate in the Ordos Basin (open sea, beach, dolomitic and gypsodolomitic flat) belong to organic facies C～D, with low hydrocarbon potential. Organic facies B can only exist in part of continental slope and part of gypsum lagoon, which is rather narrow. Organic facies study disproved the lower Ordovician carbonates to be good source rock in the Ordos Basin.