We present a multidisciplinary description of the Upper Ordovician carbonate mounds which are found throughout the central Baltic Sea and which were studied in detail on the island of Gotland. These mounds were the subject of intense exploration between 1974 and 1992 and a total of 323 shallow wells were completed in more than 100 mounds on Gotland. Many of these were put into production and a total of 100, 000 cu. m of high quality oil was produced. In this paper, we discuss the mounds' occurrence, lithological characteristics, age, faunal composition, petroleum chemistry and reservoir properties. The study is based on analyses of core material from wells in the northern part of Gotland. Upper Ordovician mound reservoirs here contain oil which has a similar geochemical signature to that in Lithuania and in the Kaliningrad district (Russia). The oil was probably derived from marine anoxic shales and migrated up-dip from more central parts of the Baltic Syneclise where oil-prone shales are known to occur. An intraformational origin for the oil is unlikely. The mounds contain large numbers of vugs and moulds which communicate mainly through dissolution fractures and surfaces and probably reflect a marine regression. Various lithofacies were recognized from petrographic studies including sub-mound, intra-mound, cap and flank, and supra-mound facies. Algae and stromatolites dominate the intra-mound facies, providing an organic framework for the entire structure. Consequently, the mounds are not merely poorly defined build-ups of mud and micrite, as has previously been assumed. Biostratigraphic data indicates a late Caradoc to Ashgill age for the mounds and their associated lithologies.