// Переписываем

TabGrid := VarArrayCreate([0,(R - 1),0,(C - 1)],VarVariant);
...
TabGrid[I,J] := FieldToVariant(dbGrid.Columns.Items[J].Field);

// И пишем функцию:

function FieldToVariant(Field:TField):OLEVariant;
begin
    Result := '';
    case Field.DataType of
         ftString, ftFixedChar, ftWideString, ftMemo,
         ftFmtMemo: Result := '''' + Field.AsString;
         ftSmallint, ftInteger, ftWord, ftLargeint, ftAutoInc:
                    Result := Field.AsInteger;
         ftFloat, ftCurrency, ftBCD: Result := Field.AsFloat;
         ftBoolean: Result := Field.AsBoolean;
         ftDate, ftTime, ftDateTime: Result := Field.AsDateTime;
    end
end;